Random thoughts and notes that will probably mostly be about colour.
by Thomas Wilshaw
This is a translation (so far done via Google Translate and very loosely proof read) of G. E. Müller’s “On Colour Sensations: Psychophysical Investigations” , published by the German Journal of Psychology and Physiology of the Sensory Organs in 1930. It is a summary of his work on colour perception consisting of over 600 pages split across two volumes.
As far as I am aware there is no English translation of this text available anywhere. This is an ongoing project and will take a lot of time. I started with the contents pages of the two volumes, partly to get a sense of what Müller covered and partly so anyone interested can request any specific sections be worked on first (my email/Mastodon account are available on my Github home page). Long term it would be nice to properly digitise/translate the entire book and upload it to the Internet Archive but I don’t even know where to begin with that.
Note on Colour names and abreviations:
Müller often reffers to colours by their initials and of course he uses the German names for the colours. For example, very early on in the text “WS-Sense” is discussed. The German translations for black and white are Schwarz and Weiß and “WS-Sense” therefore reffers to “White/Black Sense”. I decided it was going to be easier to leave these abbreviations in the original German to avoid inconsitencies. For reference a list of common colours in German is given below.
English German Abreiation White Weiß W Black Schwarz S Red Rot R Green Grün G Blue Blau B Yellow Gelb E (I believe yellow is abbreviated to E to avoid confusion with green)
After numerous transformations and adaptations to emerging new facts, I have finally brought to a certain conclusion my efforts, originally based on Hering’s views and now about 50 years old, to construct the most comprehensive and consistent theory of color perception possible. There is, however, no shortage of so-called theories of color perception. Troland occasionally gives their number as 60. The process of theory formation was usually as follows: The author would have a new idea, or at least one that appeared new to him, concerning the theory of color perception. He would then think of several facts—about 6 to 12, sometimes fewer—from this extraordinarily rich field of phenomena that seemed to agree with the emerging idea, and the theory was complete In contrast to this somewhat childish view of the nature and tasks of scientific theory formation, I present in this work theoretical views which, as the presentation should make clear, are based on a careful theoretical consideration of all the essential results of all relevant experimental investigations available to me and accordingly contribute incomparably more to the explanation of phenomena than the theories put forward thus far. I have, of course, not concealed the facts for whose explanation the theory still requires completion (cf. § 19 and § 34, IV). A general characterization of the presented theory can be found in § 64.
The subject of our investigations is color sensations, insofar as their quality and intensity are concerned. The spatial aspect of these has been disregarded. Furthermore, a discussion of the function of the rod apparatus has been omitted, as I have already dealt with it in detail in a longer treatise (Zeitschr. f. Sinnesphysiol. 54, 1923) and in a section (§§ 13-15) of my book on the types of color blindness. In view of the latter work, I was also able to refrain from a systematic treatment of color blindness, although the views to which a sufficiently thorough study of its various types leads are addressed in this work and even play a fundamental role. A discussion of the influence that attention and perceptual dispositions exert on color sensations was necessary on several occasions. In contrast, a theoretical discussion of the central brightness and color transformations, the doctrine of which is best preserved in the hands of its originator, my colleague D. Katz, has been completely omitted For the sake of brevity, some phenomena of a more peripheral nature, such as the phenomenon of the fusion frequency, which are not essential for psychophysical theory formation, have not been discussed. However, those phenomena that I have addressed due to their theoretical significance, I have endeavored to present and explain as completely as possible, taking full account of the available literature to the extent that it was accessible to me.
When Hering, the founder of the psychophysics of visual sensations and the master of experimental investigation in this field, formulated his theory of opponent colors and the physiological basis of simultaneous contrast, he naturally gave it the form in which it seemed to correspond most simply and elegantly to those phenomena that had aroused his theoretical interest. But the circle of these phenomena was very narrow compared to the totality of facts in this field that are available to us today and require explanation. While in Hering’s theory the functioning of the substances mediating the effect of light on the nerve fibers of the eye plays no essential role, we now recognize that numerous interesting phenomena in this field, some even of a dominant nature, can only be explained by referring to the peculiarities of the functioning of those light-sensitive substances. And while Hering, in his work “On the Theory of Light Sense” (p. 118), still expressed the view that the excitation processes of the three specialized optical senses proceeded independently of one another, a large part of the second volume of this work will have to deal with the influences that white light and achromatic optic nerve excitations exert on the chromatic excitation processes
Naturally, the theory to be presented here, through the closer consideration of the functioning of light-sensitive substances and the development of certain ideas about the influence of achromatic processes on chromatic ones, has acquired a considerable complexity, which makes its application somewhat difficult and contradicts the apparently existing prejudice that the theoretical treatment of visual sensations is an activity particularly suitable for dilettantes and beginners. To the objection that our theory is too complicated, I could only say that it is based on ignorance, namely, on a lack of knowledge of the highly complex nature of the extremely rich field of color sensations, which completely precludes for any thinking person the assumption that a theory of the simple character of, for example, Hering’s theory could suffice Naturally, I would welcome it as a scientific advance if someone were to prove through action that there is a theory less complicated than ours that nevertheless accomplishes the same in explaining phenomena, or that possesses the same degree of complexity as our theory and at the same time allows for an even more comprehensive explanation of phenomena.
Unfortunately, I have not been able to provide further clarification on some points (e.g., concerning colored fading and Fencher’s colors) that have only recently aroused my interest more vividly, through more extensive investigations conducted under my direction, since I have not been the director of the local Psychological Institute for about eight years. Furthermore, the condition of my eyes in recent years has prevented me from carrying out various control observations that I would not have omitted under other circumstances. I have been fortunate enough to be able to compensate for some of these points through the kindness of my colleagues HENNING, JAENSCH, KATZ, KROH, and PFAHLER, who either conducted some of the observations I requested themselves or had them carried out by members of their institutes. I would like to express my gratitude here to them, as well as to Professor ACH, who most readily made the resources of the local Psychological Institute available to me as needed. I also owe a special debt of gratitude to my colleagues KATZ and KELLER for undertaking the considerable effort of proofreading the printed sheets of this work I must also express my particular gratitude for the fact that, as a result of the kind efforts of Professor Boring, to whom I had mentioned the difficulties I was experiencing in obtaining American literature, I received no fewer than approximately 250 reprints of treatises on facial sensations by American psychologists and physiologists. While not all of these treatises, especially those dealing primarily with psychotechnical questions, could be used in the writing of this work, they nevertheless broadened my scientific horizons and reinforced my conviction that limiting oneself to the scientific literature of one’s own nation means foregoing a higher level of scientific rigor.
The fairly detailed table of contents provides information about the contents of this volume. The second volume, completed in manuscript and to a large extent already in print, will deal in detail with the aforementioned influences that achromatic processes exert on chromatic ones; furthermore, it will address simultaneous color contrast, the intrinsic brightness of colors, the electrical excitability of the visual organ, the properties of second green, and much more; and finally, it will provide a supplementary overview of the theoretical views presented, in which a position will also be taken on certain views on nerve excitation and its propagation that are widespread in physiological circles.
For the sake of simplifying citations, a number of publications that were repeatedly referenced in various chapters have been briefly listed only by their authors’ names. A list of these publications, along with some letter combinations used to cite certain journals or collected works, follows below.
A list of new or specially defined terms used in this work follows at the end of the second volume.
Göttingen, May 1930
The Author
As I have explained in more detail earlier (V, p. 12ff.), light stimuli induce three different sensitizing processes (primary processes, P-processes) in the outer cone segments, the intensities of which are determined by the wavelengths of the lights in a manner similar to the three components of the Young-Helmholtz theory. One of these processes, the P₁ process, is induced by all lights belonging to the spectral region extending from the long-wavelength end of the spectrum to approximately 475 μm. For the P₂ process, the effective spectral range extends from approximately 650 μm to at least 415 μm, and for the P₃ process from approximately 540 μm to the short-wavelength end of the spectrum. Each of these three processes has a direct white value, i.e., it acts directly on the nervous white-black substance (WS substance) in the sense of generating the excitation leading to white sensation. In addition, these three processes also act on certain chromatic sensory substances (switching substances) of the retina, a red-green and a yellow-blue substance, whose chemical transformations in turn cause the chromatic optic nerve excitations. We will discuss the function of chromatic switch substances in more detail later (§ 21). Here it may only be noted that when certain combinations of colored lights, which awaken all three parapsychological processes in certain intensity ratios, evoke the pure sensation of white, this is due to the fact that the chromatic effects of the parapsychological processes cancel each other out as a result of antagonistic relationships, so that only their immediate white values come into play. In this and the following chapters, we are generally only dealing with such light stimuli that produce a colorless sensation.
We are currently unable to say anything more definite about the way in which parapsychological processes are able to excite the nervous system substance. Weigert (p. 64ff.) distinguishes three ways in which an optical sensitizer exposed to suitable light can increase or induce a chemical process in another substance under its influence. First, the sensitizer could be induced by the light to form a substance that, as a positive catalyst, greatly accelerates a chemical process that would otherwise proceed very slowly in the added substance. Second, the light, through its action on the sensitizer, could create certain reaction nuclei which, due to the fact that the reacting substances are adsorbed onto their surfaces at a higher density, exhibit catalytic activity in relation to the reactions taking place on those substances. Finally, third, it could be a case of “catalytic coupling of a rapidly occurring photochemical reaction with another, purely chemical process”; the other chemical process is simply “carried along” by the photochemical reaction. Of these three possibilities, the first two are ruled out for our purposes here because, although catalysis can accelerate a spontaneous reaction, it cannot disturb a chemical equilibrium, such as that assumed in the case of a neutrally balanced retina. Furthermore, since a substance that promotes a specific chemical process catalytically also proves to be a positive catalyst for the opposite chemical process (Nernst, p. 663), the P-Processes, if they served to form a substance which lends greater vigor to an excitation initiated that leads to white sensation, also increase the vigor of this latter excitation in cases where the material properties of the white matter substance require the occurrence of an excitation leading to black sensation. Thus, an ongoing black excitation would have to be intensified by the action of white light
Recently, Adrian and Matthews (The Journal of Physiol., 64, 1927, pp. 279ff.) interpreted the interesting results they obtained from investigating the electromotive processes occurring in the retina and optic nerve of the eel upon exposure to light, following Hecht’s explanations, as meaning that the first effect of light in the eye is the photochemical decomposition of a substance S into the products A and P, and that the second process is a chemical reaction catalyzed by the products A+P. The preceeding shows that this view cannot be applied, at least not to the human eye.
Regarding the special case that an optical sensitizer can also mediate a work-storing chemical process that is reversed in the dark, compare Winther in the Journal of Scientific Photography 11, 1913, p. 101
It remains an open question whether the process of excitation of the WS substance by the P processes, which is of interest to us here, can be placed entirely in line with those cases that are referred to as cases of positive chemical induction.2 The positive chemical induction already known to Liebig consists in the fact that a chemical reaction taking place, for example, on substances A and B (the inducing reaction), causes or greatly accelerates a reaction taking place, for example, on substances A and C or C and D (the induced reaction), which would not have occurred anyway or would have proceeded only very slowly. It is noteworthy that there are reaction pairs where the induced reaction proceeds opposite to the affinity.
Further remarks on the functioning of the sensitizers follow on pp. 7f
We therefore assume that the occurrence of a P-process is in some way coupled with a chemical change in the WS substance (to be discussed in more detail in the next paragraph). Each P-process consists of two sub-processes together. The first partial process consists in the conversion of the photosensitive P-material from a stable first state to an unstable state by the expenditure of light energy (e.g., molecules of a certain kind undergo decomposition into specific components). The second partial process then follows, the transition from the unstable state to a second stable state. As will be shown later, we have reason to assume that this second partial process takes place with the participation of another readily available substance (the P-auxiliary). We will refer to the chemical products resulting from the first partial process as the P-intermediates and to the substances formed in the second process as the P-end products It is almost self-evident that the excitatory effect on the vitreous substance, the excitatory drive exerted upon it, originates from the second subprocess, which is a work-performing chemical process, i.e., one in which the end products have a lower energy content than the starting materials. One might assume that these are processes that occur with the consumption of oxygen, that is, that the phosphorus auxiliary substance is oxygen. I recall the observation by Wester-Lund (Skandinav. Arch. f. Physiol. 19, 1907, pp. 347ff.) that in an isolated eyeball, the action currents of the retina are suppressed by oxygen deprivation, but return after oxygen is reintroduced. But the facts to be cited further (§ 10, VI and § 49), from which it follows that the replacement of consumed P-substance occurs only at a moderate rate, must raise concerns about the assumption mentioned here.
If the views indicated above are correct, it follows that the excitation drive exerted indirectly on the WS substance by a stimulus light lasts longer than the effect of the light on the retina. For although the cessation of light exposure will simultaneously mean the cancellation of the first of the two aforementioned partial processes, which takes place using light energy, the second partial process will continue for a considerable time with decreasing intensity, since the amount of P intermediate product accumulated during light exposure is only gradually used up by conversion to P final product and partly also by regeneration into P substance. We will refer to the excitation drive arising from a light stimulus, depending on whether it is present during or after the light stimulus, as a homophotic or metaphotic excitation drive, using two terms introduced by Stigler. Since the metaphotic excitation drive must last all the longer under otherwise identical conditions the greater the amount of accumulated P intermediate product is at the moment of stimulus interruption, it follows that it persists for a longer time under otherwise identical circumstances the more intense the light stimulus was.
The important role played by the metaphotic excitation drive, especially in the case of short-duration light stimuli, is very well illustrated by an experiment by Stigler (I, p. 381ff. and II, p. 150). When he exposed one (approximately the right) half of an elliptical bright field on its own for a period of 0.05 seconds, this half shone “homogeneously in its entirety with a very considerable brightness.” However, when, at the moment this half was darkened, the other (left) half of the elliptical field was exposed to an exposure lasting the same duration of 0.05 seconds, the first exposed (right) half-field shone brightly in all its parts for a moment, only to be suddenly darkened from the boundary line upon the appearance of the second half-field, so that it appeared bright only at its outermost periphery, which was least exposed to the contrast effect of the other half-field. This experimental result can be explained by the fact that the excitation caused by a very short light stimulus, when it can develop unhindered, is, as has been observed, based to a very significant extent on the meta-photic excitation drive, and that now in the present case, the excitatory effect of this excitatory drive is largely suppressed by the contrast effect of the subsequently exposed second half-field.3 The fact that this contrast effect does not begin at the same moment the illumination of the second half-field begins only makes the contribution of the metaphotic excitation drive appear even greater.
BRÜCKNER and KIRSCH (Z. f. p. 46, 1912, pp. 280ff.) determined that the magnitude of the color time threshold depends on the gray that acts on the retinal area struck by the color immediately after it, and within certain limits, it is greater the more considerable the brightness and the duration of exposure of this gray. This finding also demonstrates the contribution that the metaphotic excitation drive has to the development of color perception in the case of short-duration colored stimuli.
The excitation drive emanating from a light of constant intensity, just as it gradually diminishes, also shows a gradual increase from the moment of light exposure. The amount of P-intermediate product gradually increases until it reaches a certain maximum value. At this maximum, the decrease it experiences through the conversion of P-intermediate product to P-final product over time equals the increase it receives through the conversion of P-substance to P-intermediate product. After reaching this maximum value, the amount of P-intermediate product decreases due to the decrease in P-substance, and then approaches zero even more rapidly after the light stimulus ceases. With very short stimulus durations, this maximum value is never reached. As long as it remains unreached, any increase in stimulus duration must also increase the duration of the metaphotic excitation drive. Once the affected area of the retina is fully adapted to the given light, a state exists in which the consumption of P-substance by the P-process is precisely compensated by the nutrient supply or regeneration of such substance.
V. KRIES occasionally remarks (I, p. 269f.) regarding the three-component theory that the assumption that the cones contain a mixture of three different light-sensitive substances may currently appear to be the most plausible of the ideas corresponding to this theory, but the question of whether these are not rather three decomposition modalities of the same body cannot be readily denied. This question, however, must be answered with certainty in the negative. For the assumption that only a single type of light-sensitive substance, albeit decomposable in three ways, is located in the cones, includes the assertion, contrary to experience, that, for example, the reduction in excitability caused by red light must apply to all other types of light in exactly the same way as to red light. For all types of colored light stimuli, the amount of available excitable material would have been reduced by the same fraction through the action of red light. Furthermore, from the standpoint of that assumption, one could only account for those differences in color systems, which are based on the fact that the three P-substances occur in different proportions in different eyes (e.g., in a normal and in a protanomalous eye), by means of rather unsatisfactory auxiliary hypotheses
The above statements (pp. 4ff.) are based on the fact, which must be rigorously demonstrated in § 10, I, that the positive afterimages are essentially of peripheral origin. This fact must be given due consideration in every theory that is formulated concerning the functioning of the sensitizers of the cone apparatus. For example, some are inclined to attribute the effectiveness of an optical sensitizer to the photoelectric effect.4 The electrons ejected from the sensitizer as a result of the light exposure would be captured by neighboring molecules or micelles and alter their structure. From the standpoint of this view, the existence of positive afterimages is justified by not considering the process caused by the electrons flung away by the sensitizer in certain molecules or micelles as the optic nerve excitation itself, but rather as a process that, like the second sub-processes of our P-processes, takes place in the receptor material, transmits an excitatory drive to the nerve optic substance, and continues for a certain time even after the light stimulus has faded.
Following quantum theory, the following explanation of optical sensitization has recently been given (NERNST, p. 908). Upon exposure to light, individual molecules of the sensitizer absorb the energy quantum hv, where h is Planck’s constant and the vibrational frequency of the absorbed light. This activates these molecules, i.e., converts them into a more energetic and reactive modification. When these activated molecules collide with molecules of the substance to be sensitized, they release this energy, making the latter more reactive. Thus, the exposure to light induces a chemical process in the substance, or an increase in the intensity of such a process, which would otherwise be absent.
If this theory is to be applied to our case of the emergence of P-processes, one must not assume that the activation of P molecules is the process that corresponds in our scheme to the conversion of P substance into P intermediate, and then assume that the conversion of P intermediate into P final product consists in the activated, particularly reactive P molecules undergoing decomposition leading to the P final product through mutual collisions with the participation of the P auxiliary substance, or leading to the P final product through energy transfer to molecules of another substance with the help of this substance. For the duration during which light-activated molecules retain their activation is far too short to account for the often minutes-long duration of a metaphotic excitation drive. “It is estimated at fractions of a hundred-millionth of a second” (NERNST, p. 897). Rather, one must assume that the process leading to the phosphorus intermediate according to our scheme consists of the activated molecules undergoing decomposition (leading to the phosphorus intermediate) upon mutual collisions, or that each phosphorus substance consists of two or more types of molecules, one of which is capable of absorbing photons and, through energy transfer, makes molecules of the other type or types more reactive and causes them to react with the formation of the phosphorus intermediate.
Since light can only have chemical effects in a substance to the extent that it is absorbed by that substance, the appearance of the retina under objective observation in mixed colorless light must depend on the amounts of the three P substances present in the outer segments of the cones, each of which has its own particular absorption spectrum, provided these substances are not present in excessively high dilution. With a neutral retinal state, the light absorptions of the three P substances will be approximately balanced, for example, red light being absorbed by the P₁ substance and green light by the P₂ substance, so that no noticeably coloring effect on the retina results. On the other hand, as Tscher-Mak (II, p. 583) has emphasized, a permanent color shift could, under favorable conditions, lead to an objectively detectable discoloration of the retina in the direction of that same color. However, the absorption capacity of the resulting detuning products could play a certain role here.
One cannot expect that, in the resting state, the P-substances are found in the outer segments of the cones in a similar concentration to that of rhodopsin in the resting outer segments of the rods. For, as Hering (II, p. 114) has already explained in more detail, such a sensory apparatus requires, in order to perform certain functions, a smaller supply of light-sensitive material the more actively the nutritive replacement of consumed material takes place within it. In the rods, this replacement occurs at a relatively slow rate. Therefore, in a fully recovered state, prepared for new achievements, they show an abundant supply of purple, betrayed by the corresponding coloration of the retina. In the cones, however, a much more active replacement of the used-up light-sensitive material takes place. Therefore, one cannot expect the same resting supply of light-sensitive material in them.
If, with a neutral state of the retina, a P-process is induced by acting light, this process has the following effect on the WS substance: firstly, that components of a certain A-material (starting material) are transformed into another material, which may be called the W-material for short. Secondly, it also simultaneously causes components of this latter material, whether with or without the participation of other readily available substances, to be transformed into components of a third material, essentially different from the A-material, which may be called the S-material for short5.
Thus, despite this terminology, we are dealing with 3 materials in the WS substance: the A-material, the W-material, and the S-material Between these, four chemical reactions take place: an AW reaction (conversion of A material into W material), a WA reaction (the opposite conversion), a WS reaction, and a SW reaction. The excitation drive exerted on the WS substance by an occurring P process, and which may be referred to briefly as the W drive, acts in the sense of increasing the AW reaction and weakening the WA reaction, and simultaneously also in the sense of increasing the WS reaction and weakening the SW reaction. An S drive has precisely the opposite effects, such as that which occurs when an electric current flows through the visual organ in a downward direction and also exists for an almost lightless field exposed to the darkening contrast effect of a white environment. If we refer to the overall process consisting of the AW and WS conversions as the AS conversion and the overall process consisting of the SW and WA conversions as the SA conversion, we can briefly say that a W drive promotes the AS conversion and an S drive promotes the SA conversion in its two sub-processes.
It is advisable to characterize the modes of action of a W and an S drive described above in more detail with reference to the law of chemical mass action. If the dependence of the rate of AW conversion on the factors determining it is represented by a formula in accordance with this law, the velocity coefficient, which may be denoted as KAW for short, appears in this formula as the quantity that expresses the influence of the W drive on the AW conversion. Likewise, the values of the velocity coefficients KWA, KWS, and KSW express the influence of the existing W drive on the WA, WS, and SW conversion. The stronger this drive is, the larger the values of KAW and KWS and the smaller those of KWA and KSW. The occurrence or strengthening of an S drive manifests itself in the opposite direction
Since an increase or decrease in KAW is always associated with a decrease or increase in KWA, and vice versa, we will generally limit ourselves in the following discussion to the behavior of only one of these two coefficients. The behavior of the other is then readily apparent. The same applies to the coefficients KWS and KSW.
According to the above, the state of a WS substance under the influence of a W or S drive, insofar as it concerns the conversions mentioned so far, is characterized by two things: firstly, by the values that the quantities of A, W, and S material possess at the given moment, corresponding to the preceding effectiveness of the W or S drive; and secondly, by the values that the four rate coefficients mentioned above have according to the current magnitude of the W or S drive. If, for example, a light stimulus is interrupted after a certain duration, a significant quantity of S material has accumulated as a result of the acceleration that the AW and WS conversion experienced through the W drive. For the material transformations now occurring within the WS substance, however, not only are the quantitative ratios of the three materials involved decisive, but also the meta-photic values of the four rate coefficients.
If we consider the process that a W-driven impulse causes in the WS substance as consisting of two sub-processes, an oxidation reaction and a AW-driven reaction, this is not a peculiar assumption on our part, but corresponds to the common occurrence that a chemical substance in a stable state is transformed by a process acting upon it, e.g.radiation, into another stable substance in such a way that it is first converted into a different material constellation with a higher energy content, from which it then spontaneously (possibly also with simultaneous acceleration by the acting process) transitions into that second stable state. Similarly, we assume that the components of the W material are unstable material constellations that possess a higher energy content than the components of the A and S materials, and that accordingly, the AW conversion and the SW conversion, which take place under the influence of the work-performing second sub-processes of the P processes, are work-storing processes, whereas the WA and WS conversions are work-performing processes. Of the latter two conversions, the former receives a certain boost from an existing S drive, and the latter from an existing W drive.
One would expect the processes propagating in the visual pathway, or at least in certain parts thereof, to be processes in which energy is made available through the release of tension forces to enable the occurrence of a transmission stimulus acting on the next cross-section. Physiological concerns have rightly been raised against the assumption of the propagation of purely assimilatory processes within a nerve pathway. Our theory corresponds to the expectation mentioned here, since according to it, both when a W-drive and when an S-drive are present, the second of the two occurring subprocesses is a work-performing process
Turning now to the question of the psychophysical significance of the transformations taking place in the WS substance, the following must be said. The WS and SW transformations, as opposing processes, cancel each other out in their effectiveness, so that only the difference in intensity between the two transformations, which represents the effective transformation resulting in one direction or the other, is psychophysically effective in the indirect manner to be described below. The same applies to the AW and WA transformations. Only the effective WA or AW transformation represented by the difference in intensity between the WA and AW transformations is psychophysically effective.
Psychophysical W-excitation and S-excitation are distinct from the processes of the WS substance. To grasp the psychophysical efficacy of the aforementioned effective processes, it is necessary to consider more closely the unique characteristic of the visual sense, which consists in the fact that even in the absence of any stimulus, a sensation of this specialized sense—the sensation of subjective ocular gray, which encompasses both whitishness and blackness—is present. This sensation is based on the fact that even when neither a W- nor an S-drive acts upon the optic nerve, an endogenous excitation exists in the psychophysical visual sphere. This excitation consists of a coexistence and intermingling of psychophysical processes, some of which are effective in and of themselves in generating a W-sensation, while others are effective in arousing an S-sensation. I have already discussed (II, pp. 40ff.) the facts that show that the perception of subjective ocular gray is not based on retinal processes but is of central origin. The main emphasis should be placed on the occurrence of cases (positive scotomas, hemianopic black vision, pressure blindness) where the transmission of peripheral stimuli (at least those of the intensity under consideration here) to the more central parts of the visual pathway is completely excluded, and the subjective gray of the eyes is still perceived.
Regarding that endogenous excitation of the visual organ, it is reasonable to assume that it is based on the internal thermal motion occurring within the visual substance. As a result of this internal thermal motion, opposing chemical reactions with equal vigor are constantly taking place in every material system that is in so-called chemical equilibrium. Accordingly, in the central visual substance left to its own devices, AW, WA, WS, and SW reactions are constantly occurring side by side, to which a sensation imbued with both whitishness and blackness naturally corresponds. The substrates of chromatic excitations are more resistant to internal thermal motion; they are not so easily determined by it to the reactions that can be realized by it and are also present in smaller quantities in the central visual substance, so that the sensation of the subjective gray of the eye lacks distinct colored admixtures. The view suggested here is untenable because, if it were correct, we would be familiar with red-green and yellow-blue sensations despite the claimed higher resistance of chromatic substrates to the influence of internal thermal motion. If, for example, we allow red light to act on the same area of the retina for an extended period, an ever-increasing amount of G-material accumulates in the relevant visual substance through the conversion of R-material (red material) into G-material (green material)6. Thus, alongside the gradually decreasing conversion of R-material to G-material, the opposite conversion takes place to an increasing degree. If the AS and SA conversions, or certain opposing subprocesses thereof, e.g. if the WS and SW conversions together served as the basis for a black-and-white mixed sensation, then these two conversions in the area of red-green vision should also evoke a corresponding red-green sensation. This is by no means the case, however; rather, the two opposing processes counteract each other with regard to their psychophysical effectiveness, so that under certain circumstances only an almost colorless sensation remains, which, after the removal of the red light, is very soon followed by a very pronounced green afterimage, which sufficiently testifies to the extent of the preceding accumulation of G-material. The transition from the afterimage of the same color to the greenish-red one would also have to be characterized, if the two opposing processes did not compensate for each other, not by the absence of a sensation of red-greenness, but by a sensation of distinct red-greenness. The same applies to the processes of the yellow-blue sense. We must therefore assume that the AS and SA processes also counteract each other with their subprocesses in a psychophysical sense, and that the sensation of subjective eye gray is not based on their coexistence. The analysis presented here would also apply if one were to adopt Hering’s theory of assimilation and dissimilation processes. If the antagonism of assimilation and dissimilation excludes red-green and yellow-blue sensations, then this antagonism must also preclude, in the area of the WS sense, that assimilation and dissimilation processes jointly serve as the basis for a corresponding mixed sensation, and that the sensation of subjective ocular gray is based on a coexistence of these antagonistic processes.
To overcome the difficulty at hand, I see no other way than to assume that the ultimate basis of W and S sensations is not the WS and SW conversions, but rather two qualitatively different and non-opposing processes, which can be briefly described as W and S excitation, and of which the former is promoted by an effective WS conversion and reduced by an effective SW conversion, while the latter, S excitation, is promoted by an effective WA conversion and adversely affected by an effective AW conversion7. Both processes are without noticeable inertia or fatigue, and their products, subject to elimination, have no noticeable influence on the further course of excitation8. Thus, apart from the endogenous factor to be mentioned immediately, the respective W or S excitation is essentially determined only by the current direction and vigor of the effective exchange between the W and S material or between the W and A material. However, as just indicated, these effective exchanges are not the only factors determining the two psychophysical excitations of the visual system. Even at rest, W and S excitations occur continuously, perhaps as a result of internal heat motion. These excitations form the basis of the subjective sensation of the gray of the eye and undergo quantitative changes under the influence of external factors, according to the effective exchanges developing in the visual system. These effective exchanges are, however, as just indicated, not the only factors determining the two psychophysical excitations of the visual system. The AW and WA transformations, as well as the WS and SW transformations, naturally occur to some extent even at rest in the visual substance due to internal thermal motion. However, if the first two and the latter two opposing transformations are of equal intensity, they cancel each other out with regard to their effect on W and S excitation, so that these two psychophysical excitations only manifest with their endogenous values, unaffected by any processes of the WS substance.
Based on the above, the process that takes place when white light strikes the visual organ, which is in a neutral state, can be described as follows. The W-drive, which arises from the P-processes that occur, acts on the one hand by promoting AW and inhibiting WA turnover, thus increasing W-material, and on the other hand, it increases the ease with which W-material is converted into S-material, so that it manifests itself in two ways in terms of increasing WS turnover. Thus, an effective WS turnover occurs, which is greater the stronger the W-drive. However, it should not be overlooked that the more intense the effective WS turnover, the greater the amount of S-material that accumulates during the excitation, despite the removal of substances. This in itself promotes S-material turnover and negatively influences the effective WS turnover. This influence of the increase in S-material is counteracted by the fact that the W-drive, according to its strength, simultaneously inhibits the SW-transaction. The effective WS-transaction, according to its magnitude, then causes an intensification of the psychophysical W-excitation, which is already present as a component of endogenous excitation. At the same time, the AW-transaction caused by the W-drive acts in the sense of reducing the already existing psychophysical S-excitation. Accordingly, the sensation must gain in whiteness and lose in blackness. If, according to a remark by Donders (Arch. f. Ophth. 30, I, p. 47), from a certain degree of white light intensity onward the white sensation no longer seems to have any blackness in it, it remains doubtful whether this is due to a real suppression of S-excitation by the existing W-drive or merely to an insufficient value of its psychophysical weight.
If an S-drive acts on the visual substance of the resting eye, it manifests itself by promoting SW and WA turnover and weakening WS and AW turnover in the sense of increasing S excitation and decreasing W excitation, so that, according to its strength, the sensation gains in blackness and loses in whiteness compared to the sensation of the subjective gray of the eye.
If we allow an acting white light to become progressively stronger, starting from a minimal intensity, not only does the quality of the sensation change—its whiteness increases and its blackness decreases—but the intensity and penetrating nature of the sensation also increase. The resulting increase in the overall intensity of psychophysical excitation is based on the fact that, when a W-drive is applied, the resulting increase in the intensity of the W-excitation is greater than the simultaneous decrease in the intensity of the S-excitation. This behavior is likely related to the fact that the weakening of S-excitation can only go down to zero, while no corresponding limit is set for the increase of W-excitation. For the same reason, when an S-drive is applied, the growth of the S-excitation is greater than the decrease of the W-excitation, so that in this case, in addition to the increase in the blackness and reduction in the whiteness of the sensation, there is also an increase in the intensity and vividness.
From the above, it follows that the sensation present when neither a W-drive nor an S-drive acts on the nerve optic is the link in the black-and-white sensation series that possesses the lowest intensity. It is called the sensation of critical gray.
Some have argued that the black and white sensations do not represent any differences in intensity at all. This view is greatly exaggerated. For the sensation of a bright white or a black that is greatly deepened by contrast is undoubtedly to be attributed a higher intensity than the sensation of the subjective gray of the eye or a sensation closely related to it. However, it must be admitted that the differences in intensity of the sensations caused by white lights of varying strength under the same conditions are relatively small, which is due on the one hand to the slow increase in W-excitation with increasing stimulus and on the other hand to the fact that an increase in W-excitation is accompanied by a weakening of S-excitation. I previously measured the increase in the intensity of the corresponding achromatic excitation that occurs when a W or S stimulus is intensified by the significant increase in the color threshold that occurs when a W or S stimulus acting alongside the colored stimulus is intensified, assuming that this increase in the color threshold was solely due to the increase in the psychophysical excitation corresponding to the W or S stimulus. In fact, however, the increase in the color threshold, insofar as it occurs when a W stimulus simultaneously strikes the same retinal area is intensified, is partly due to the antichromatic influence of white, which will be discussed in more detail later (§ 39), and, insofar as it is due to an increase in a simultaneous S stimulus, partly due to the decrease in the amount of W material, which is detrimental to chromatic excitation (cf. § 41).
It hardly needs to be recalled that the subjective visual field, which presents itself after a prolonged period of darkness during which all afterimages of preceding stimuli have been extinguished, generally does not only exhibit black and white content based on the aforementioned endogenous W and S excitation, but often presents cloud-like formations and other, not infrequently colored, phenomena caused by internal stimuli. Especially in the case of blind people, such subjective phenomena, which are supposedly also dependent on the weather, often play a major role9. J. Plateau reported that even 40 years after his complete blindness, when he focused his attention on his eyes, he still received sensations of varying brightness and sometimes slightly reddish shades of gray. Naturally, under these circumstances, the perception of critical gray cannot simply be identified with the perception of subjective eye gray. It is identical to this only under the condition that the visual substance is not influenced by any internal stimuli.
The objection often raised against the opponent-color theory is that, just as red-green and yellow-blue sensations do not occur (a claim that, as we shall see later [§27], can only be made with such absolute certainty), according to this theory gray sensations should also not exist. The gray sensations occur because every W-drive (S-drive), as long as it is unable to suppress the S-component (W-component) of the endogenous excitation, must necessarily cause a W-excitation (S-excitation) accompanied by S-excitation (W-excitation). In contrast, an R-drive encounters no noticeable G-component of the endogenous excitation. Therefore, even at low intensity, it essentially corresponds only to an R-excitation and a red sensation not associated with noticeable greenishness. The same applies to a G-, E- or B-drive.
The question arises as to how far the endogenous excitation of the visual organ and the provision of the visual substance with the substrates underlying psychophysical W and S excitation extend peripherally. Hering (Hermann’s Handbook of Physiology, 1, p. 568) did not rule out the possibility that the psychophysical substance extends as far as the retina and that sensation enters consciousness as the joint product of the retina and the brain, or “as a unified correlate of the process distributed between the retina and the brain”. We can now rule out this possibility. For on the one hand, we now know (cf. § 5) that the zone in which the optic nerve excitations cause the processes of simultaneous contrast lies beyond Gratiolet’s optic radiation. On the other hand, as I have shown earlier (V, p. 23ff.), there is no doubt that there are cases of green blindness with preserved red sensation in which pure green light appears pure gray, but nevertheless evokes the red contrast color in the surroundings. In these cases, therefore, when exposed to green light, the optic nerve excitation corresponding to green is present even in the contrast zone located behind the green optical radiation, without this excitation being able to influence the quality of the resulting color sensation. Accordingly, we must place the boundary of the psychophysical processes of the sense of sight in a zone that lies beyond the contrast zone located behind Gratiolet’s optic radiation.
I would like to mention that one can do justice to the facts as well as to the views developed above by taking as a basis the opinion contained in the following 3 sentences.
An effective WS turnover promotes psychophysical W excitation and disadvantages S excitation, both of which are already present as components of endogenous excitation.
Effective SW turnover promotes psychophysical S excitation and inhibits W excitation.
Effective WA or AW transform has no effect on either of these psychophysical excitations.
According to this view, W excitation, just as in the view presented above, is promoted by an effective WS turnover and suppressed by an effective SW turnover.
Regarding the achromatic excitations of the rod apparatus, compare my significant earlier explanations (V, p. 118ff.) ↩
Compare, regarding the same, the instructive treatise by A. Skrabal, “The Induced Reactions, Their History and Theory”. Collection of Technical and Chemical-Technical Lectures, edited by S. B. Ahrens and W. Herz, Volume 13, Stuttgart 1908. ↩
Stigler refers to the contrastive influence on the effect of a metaphotic excitatory drive by the simultaneous stimulation of an adjacent retinal area as metaphotic contrast or metacontrast ↩
Compare, for example, F. SCHANZ in Pflügers Archiv 190, 1921, p. 317 ↩
It is irrelevant that we disregard here the possibility that in the conversion of A-material to W-material such conversion products are also formed which are of course involved in the regeneration of W-material to A-material, but not also in the conversion of W-material to S-material ↩
More details on chromatic optic nerve excitations can be found in § 22. In § 27 we will further see that, according to our theory, while the frequency and commonality of red-green and yellow-blue sensations is absolutely excluded, as explained above, their isolated occurrence under special circumstances is not. ↩
Tschermak (II, p. 488f. and 580) also came to the conclusion that while the preterminal excitations corresponding to white and black must be considered antagonistic, the terminal excitations directly underlying the whitishness and blackness of sensations, especially also the subjective gray of the eyes, must not be regarded as opposite to each other. ↩
An example of the occurrence of processes of the type mentioned here are electrical oscillations, which in themselves—that is, insofar as they do not act on other substrates—leave no excitation products whatsoever. ↩
See v. GERHARDT, Materials on the Psychology of the Blind, Langensalza 1917, pp. 112ff. ↩