THE INFLUENCE OF TEMPERATURE ON THE PHOTOSENSORY LATENT PERIOD

1. The effect of temperature on the photosensory latent period in Pholas dactylus is accurately described by the Arrhenius equation when µ = 18,300. 2. The adequacy of this equation has already been found for two other photosensitive animals, Mya and Ciona, which are very similar in behavior to Pholas. The value of µ is different for each of the three species studied. 3. This is taken to mean that though the organization of the receptor process is the same for the three species, the chemical materials concerned are very likely different.     

THE PHOTOCHEMICAL BASIS OF ANIMAL HELIOTROPISM

1. Experiments on the heliotropic orientation of Limulus were made which confirmed Loeb''s photochemical theory of animal heliotropism proposed first in 1888 and 1889 in experiments on insects, and later in experiments on other forms of animals. 2. It is shown that these animals are oriented by light in such a way that the product I x t x cos α is the same for the symmetrical photosensitive elements of the eyes or the skin, where I is the intensity of the light, t the duration of illumination, and α the angle of incidence of the light at the surface element of the photosensitive organ. 3. When this equation holds, the products of decomposition by light must be the same in symmetrical elements of the eyes or skin, and the influence of these products of decomposition on the tension of symmetrical muscles of the locomotor organs of the animal must be the same. As a consequence the animal must move in the path of light, either to or from the source of light.     

THE EFFECTIVENESS OF THE SPECTRUM IN CHLOROPHYLL FORMATION

1. Although the carotenoid pigments are present in large concentration in the plastids of etiolated Avena seedlings as compared with protochlorophyll, the pigment precursor of chlorophyll, it is possible to show that the carotenoids do not act as filters of the light incident on the plant in the blue region of the spectrum where they absorb heavily. This suggests that the carotenoids are located behind the protochlorophyll molecules in the plastids. 2. Since the carotenoids do not screen and light is necessary for chlorophyll formation, an effectiveness spectrum of protochlorophyll can be obtained which is the reciprocal of the light energy necessary to produce a constant amount of chlorophyll with different wavelengths. The relative effectiveness of sixteen spectral regions in forming chlorophyll was determined. 3. From the effectiveness spectrum, one can conclude that protochlorophyll is a blue-green pigment with major peaks of absorption at 445 mµ, and 645 mµ, and with smaller peaks at 575 and 545 mµ. The blue peak is sharp, narrow, and high, the red peak being broader and shorter. This differs from previous findings where the use of rougher methods indicated that red light was more effective than blue and did not give the position of the peaks of absorption or their relative heights. 4. The protochlorophyll curve is similar to but not identical with chlorophyll. The ratio of the peaks of absorption in the blue as compared to the red is very similar to chlorophyll a, but the position of the peaks resembles chlorophyll b. 5. There is an excellent correspondence between the absorption properties of this "active" protochlorophyll and what is known of the absorption of a chemically known pigment studied in impure extracts of seed coats of the Cucurbitaceae. Conclusive proof of the identity of the two substances awaits chemical purification, but the evidence here favors the view that the pumpkin seed substance, which is chemically chlorophyll a minus two hydrogens, is identical with the precursor of chlorophyll formation found in etiolated plants.     

THE PHOTOTROPIC SENSITIVITY OF PHYCOMYCES AS RELATED TO WAVE-LENGTH

Under the circumstances of experimentation described, the sporangiophores of Phycomyces are found to be most sensitive to stimulation by light in the violet between 400 and 430 mµ. Toward the red, sensitivity falls to nearly zero near 580 mµ, while in the near ultra-violet around 370 mµ, sensitivity is still high. The previous experiments of Blaauw had placed the point of greatest sensitivity some 80 mµ nearer the red end of the spectrum. Because of the known presence in the sporangiophores of Phycomyces of "accessory" pigments, care must be taken in identifying such results with the absorption spectrum of the photosensitive substance.     

THE CHLOROPHYLL-PROTEIN COMPOUND OF THE GREEN LEAF

1. Aqueous extracts of spinach and Aspidistra leaves yield highly opalescent preparations which are not in true solution. Such extracts differ markedly from colloidal chlorophyll in their spectrum and fluorescence. The differences between the green leaf pigment and chlorophyll in organic solvents are shown to be due to combination of chlorophyll with protein in the leaf. 2. The effect of some agents on extracts of the chlorophyll-protein compound has been investigated. Both strong acid and alkali modify the absorption spectrum, acid converting the compound to the phaeophytin derivative and alkali saponifying the esterified groups of chlorophyll. Even weakly acid solutions (pH 4.5) denature the protein. Heating denatures the protein and modifies the absorption spectrum and fluorescence as earlier described for the intact leaf. The protein is denatured by drying. Low concentrations of alcohol or acetone precipitate and denature the protein; higher concentrations cause dissociation liberating the pigments. 3. Detergents such as digitonin, bile salts, and sodium desoxycholate clarify the leaf extracts but denature the protein changing the spectrum and other properties. 4. Inhibiting agents of photosynthesis are without effect on the absorption spectrum of the chlorophyll-protein compound. 5. The red absorption band of chlorophyll possesses the same extinction value in organic solvents such as ether or petroleum ether, and in aqueous leaf extracts clarified by digitonin although the band positions are different. Using previously determined values of the extinction coefficients of purified chlorophylls a and b, the chlorophyll content of the leaf extracts may be estimated spectrophotometrically. 6. It was found that the average chlorophyll content of the purified chloroplasts was 7.86 per cent. The protein content was 46.5 per cent yielding an average value of 16.1 parts per 100 parts of protein. This corresponds to a chlorophyll content of three molecules of chlorophyll a and one of chlorophyll bfor the Svedberg unit of 17,500. It is suggested that this may represent a definite combining ratio of a and b in the protein molecule.     

THE KINETICS OF DARK ADAPTATION

1. Data are presented for the dark adaptation of four species of animals. They show that during dark adaptation the reaction time of an animal to light of constant intensity decreases at first rapidly, then slowly, until it reaches a constant minimum. 2. On the assumption that at all stages of adaptation a given response to light involves a constant photochemical effect, it is possible to describe the progress of dark adaptation by the equation of a bimolecular reaction. This supposes, therefore, that dark adaptation represents the accumulation within the sense cells of a photosensitive material formed by the chemical combination of two other substances. 3. The chemical nature of the process is further borne out by the fact that the speed of dark adaptation is affected by the temperature. The velocity constant of the bimolecular process describing dark adaptation bears in Mya a relation to the temperature such that the Arrhenius equation expresses it with considerable exactness when µ = 17,400. 4. A chemical mechanism is suggested which can account not only for the data of dark adaptation here presented, but for many other properties of the photosensory process which have already been investigated in these animals. This assumes the existence of a coupled photochemical reaction of which the secondary, "dark" reaction is catalyzed by the products of the primary photochemical reaction proper. This primary photochemical reaction itself is reversible in that its main products combine to form again the photosensitive material, whose concentration controls the behavior of the system during dark adaptation.     

STUDIES ON BIOLUMINESCENCE : IX. CHEMICAL NATURE OF CYPRIDINA LUCIFERIN AND CYPRIDINA LUCIFERASE.

There seems to be very little doubt but that luciferase is a protein or so closely associated with proteins that their removal destroys its characteristic properties. The particular group of proteins to which it belongs may be arrived at by a process of exclusion, and only the group of albumins has properties which agree completely with those of luciferase. Dubois believes Pholas luciferase to be an oxidizing enzyme similar to the oxydones of Battelli and Stern because it is readily destroyed by fat solvents such as chloroform, strong alcohol, etc. He has detected iron in a luciferase solution which has dialyzed against running water for a long time, and believes it to be made up of protein in combination with iron and to act as an "oxyzymase ferrique." Cypridina luciferase, on the other hand, is not readily destroyed by fat solvents. Toluene and chloroform are good preservatives, and I often make use of them for this purpose, keeping the luciferase solutions for many months. Professor A. H. Phillips of Princeton University has very kindly analyzed some whole dried Cypridinœ for me and finds iron, copper, and manganese but no zinc or vanadium to be present. Whether these metals are connected with the action of Cypridina luciferase is uncertain, but it is significant that all three of the metals thought to be concerned in organic oxidations are present. Although a large amount of luciferin mixed with a small amount of luciferase will use up all the latter, I agree with Dubois that luciferase has sufficient properties in common with the enzymes as a class to be considered an enzyme. The peroxidases are well known to be used up in the reactions they accelerate. All workers on enzymes agree that the more enzymes are purified the less active they become. The chemical procedures necessary to remove foreign material bring about irreversible changes in the enzyme itself, a characteristic also of many protein groups and of the colloidal state in general. This is true in the case of luciferase, for the crude luciferase solution is the most active preparation that can be obtained. I believe that Cypridina luciferase should be placed in a class of oxidizing enzymes by itself—a group having the chemical reactions of an albumin, possibly in combination with some heavy metal, and which as far as we know, acts specifically on only one substance, Cypridina luciferin. It resembles the plant peroxidases in resisting the action of chloroform, toluene, etc., but will not oxidize any of the hydroxyphenol or aminophenol compounds so readily oxidized by the peroxidases, nor will the peroxidases oxidize luciferin with light production. Dubois'' researches show that Pholas luciferase differs in some properties from Cypridina luciferase, and my own work indicates that firefly luciferase is more like that of Pholas. A comparative study of other species of luminous animals is needed in order to delimit more accurately the class of luciferases as a whole. Luciferin presents many characteristics in common with the proteins, but two, which, to say the least, throw doubt on its protein nature: (1) its peculiar solubility (in alcohols, esters, and glacial acetic acid), (2) and its resistance to digestion by proteases, even by trypsin which has almost universal digestive action. These two peculiarities have been discussed above. We can only say that if a protein, luciferin must belong to a new group differing from known natural proteins in these respects. In general characteristics this new group would fall somewhere on the border-line between the proteoses and peptones. It would not be surprising to find in nature proteoses or peptones soluble in absolute alcohol. We know also that some NH-CO linkages of proteins are broken down with great difficulty by trypsin as it is difficult to obtain a tryptic digest of protein which does not give the biuret reaction, and the work of Fischer and Abderhalden has shown that certain artificial polypeptides are not digested by pure activated pancreatic juice. We have, then, three possibilities. Luciferin is (1) either a natural proteose not attacked by trypsin, or (2) if attacked by trypsin, its decomposition products (presumably amino-acids) still contain the group oxidizable with light production, or (3) it is not a protein at all. I believe that luciferin has too many protein characteristics to conform to the last possibility. I have been unable to oxidize with light production various mixtures of amino-acids (from beef and casein) by means of luciferase and consequently am led to believe that luciferin is a new natural proteose, soluble in absolute alcohol and not digested by trypsin. Dubois believes Pholas luciferin to be a natural albumin with acid properties. Cypridina luciferin could not possibly be regarded as an albumin, but it is very likely that the luciferins of different species of luminous animals differ in certain characteristics. As in the case of the luciferases, we know that the luciferins are not identical substances, and only future work can determine in what particulars they differ. A summary of the properties of Cypridina luciferin and Cypridina luciferase will be found in the tables accompanying this paper.     

THE CHEMISTRY OF DAYLIGHT VISION

1. While several reports of photosensitive pigments from the retinas of animals possessing large numbers of cone cells have been published, the only study which could be confirmed was Wald''s discovery of iodopsin, a red-sensitive pigment from chicken eyes. 2. In its chemical properties, such as the range of pH stability and the effect of polar organic solvents, iodopsin resembles rhodopsin but is considerably more labile. 3. A partial purification from inert yellow impurities has been effected by prehardening the retinas in pH 4.9 acetate buffer before extraction by 2 per cent digitonin. Rhodopsin was an inevitable contaminant in most methods of extraction, but could be reduced to about 10 per cent of the absorption due to iodopsin by extraction of unhardened retinas with 4 per cent Merck''s saponin in ¾ saturated magnesium sulfate for about 1 hour. 4. The rate of bleaching of iodopsin was found to be first order and linear with respect to energy. 5. The bleaching effectiveness spectrum of iodopsin was determined with the aid of color filters of known energy transmission, and shows a maximum at 560 mµ in the yellow green with a lower plateau in the blue. The spectrum is in good agreement with the sensitivity of the human cones except for the wavelength of maximum bleaching effectiveness. The maximum sensitivity of the human cones is found at 540 mµ. 6. Previous reports of changes in pH and inorganic phosphate level of retinas due to bleaching could not be confirmed.     

MICROSPECTROPHOTOMETRY AND THE PHOTORECEPTOR OF PHYCOMYCES I

By applying microspectrophotometry to the sporangiophore of Phycomyces blakesleeanus wild-type and the albino car-10(-) type II, absorption spectra were obtained for 1- to 5-day cultures. Spectra in the growing-zone of the wild-type during Stage IVb, taken from 0.1 to 3 mm below the base of the sporangium, show two distinctly different spectra: one is more characteristic of a carotene, the other of a flavin. Combined, these absorption spectra reproduce closely the action spectrum. For the albino car-10(-), which is deficient in carotenes, only the spectrum characteristic of lumichrome or a reduced flavin was found. A c-type cytochrome was isolated from both strains which, if coupled with a flavin, could permit a photoreversible oxidation-reduction system. Birefringent crystals were observed to be aligned in the growing zone in which the photoreceptor is believed to lie. Micro-spectrophotometry of these crystals shows absorption peaks similar to those of riboflavin crystals.     

Events Surrounding the Early Development of Euglena Chloroplasts: VI. Action Spectra for the Formation of Chlorophyll, Lag Elimination in Chlorophyll Synthesis, and Appearance of TPN-dependent Triose Phosphate Dehydrogenase and Alkaline DNase Activities

Action spectra derived from dose-response curves measured for various processes associated with chloroplast development in Euglena gracilis var. bacillaris are presented. The action spectrum for chlorophyll synthesis during the first 36 hours of continuous illumination of dark-grown resting cells resembles the absorption spectrum of protochlorophyll(ide). The action spectrum for the preillumination phase of potentiation, during which preillumination followed by a dark period brings about lag elimination in chlorophyll synthesis when the cells are subsequently exposed to postilluminating light, shows a high peak in the blue region (at about 433 nm) with a small peak in the yellow-orange region (at about 597 nm); the postillumination phase yields an action spectrum very similar to that obtained for chlorophyll synthesis in continuous light in normal, unpotentiated cells, with peaks at 433 and 631 nm. Alkaline DNase and TPN-linked triose phosphate dehydrogenase, two plastid enzymes which are synthesized outside the chloroplast, yield action spectra which are consistent with protochlorophyll(ide) being the major light receptor. The action spectra which implicate pigments resembling protochlorophyll(ide) holochrome have blue to red peak ratios in the vicinity of 5:1 as does the absorption spectrum of the protochlorophyllide holochrome from beans; the action spectrum is not identical with the holochrome spectrum indicating that the Euglena holochrome may differ from the bean pigment in details of its absorption spectrum. The action spectrum for preillumination, shows a ratio of the blue peak to the red effectiveness of about 24:1. This suggests that preillumination is controlled by a photoreceptor different from the protochlorophyll(ide) holochrome.     

A STUDY OF THE BACTERICIDAL ACTION OF ULTRA VIOLET LIGHT : III. THE ABSORPTION OF ULTRA VIOLET LIGHT BY BACTERIA

The simple conclusion of former investigators that the shorter the wave length of ultra violet light the greater the bactericidal action is in error. A study with measured monochromatic energy reveals a characteristic curve of bactericidal effectiveness with a striking maximum between 260 and 270 m.µ. The reciprocal of this abiotic energy curve suggests its close relation to specific light absorption by some single essential substance in the cell. Methods are described for determining the absorption curve, or absorption coefficients, of intact bacteria. These curves for S. aureus and B. coli have important points of similarity and of difference with the reciprocals of the curves of bactericidal incident energy, and point the way in a further search for the specific substance, or substances, involved in the lethal reaction.     

The nature of the gecko visual pigment

Retinal extracts of the Australian gecko, Phyllurus milii (White), have revealed the presence of a photosensitive pigment, unusual for terrestrial animals, because of its absorption maximum at 524 mµ. This pigment has an absorption spectrum which is identical in form with that of other visual chromoproteins. It is not a porphyropsin, for bleaching revealed the presence, not of retinene₂, but of retinene₁ as a chromophore. Photolabile pigments with characteristics similar to those of the Phyllurus visual pigment were also detected in retinal extracts of six other species of nocturnal geckos. The presence of this retinal chromoprotein adequately accounts for the unusual visual sensitivity curve described by Denton for the nocturnal gecko. This pigment may have special biological significance in terms of the unique phylogenetic position of geckos as living representatives of nocturnal animals which retain some of the characteristics of their diurnal ancestors. The occurrence of this retinene₁ pigment, intermediate in spectral position between rhodopsin and iodopsin, is interpreted in support of the transmutation theory of Walls. The results and interpretation of this investigation point up the fact that, from a phylogenetic point of view, too great an emphasis on the duplicity theory may serve to detract attention from the evolutionary history of the retina and the essential unitarianism of the visual cells.     

THE VISIBILITY OF MONOCHROMATIC RADIATION AND THE ABSORPTION SPECTRUM OF VISUAL PURPLE

1. After a consideration of the existing data and of the sources of error involved, an arrangement of apparatus, free from these errors, is described for measuring the relative energy necessary in different portions of the spectrum in order to produce a colorless sensation in the eye. 2. Following certain reasoning, it is shown that the reciprocal of this relative energy at any wave-length is proportional to the absorption coefficient of a sensitive substance in the eye. The absorption spectrum of this substance is then mapped out. 3. The curve representing the visibility of the spectrum at very low intensities has exactly the same shape as that for the visibility at high intensities involving color vision. The only difference between them is their position in the spectrum, that at high intensities being 48 µµ farther toward the red. 4. The possibility is considered that the sensitive substances responsible for the two visibility curves are identical, and reasons are developed for the failure to demonstrate optically the presence of a colored substance in the cones. The shift of the high intensity visibility curve toward the red is explained in terms of Kundt''s rule for the progressive shift of the absorption maximum of a substance in solvents of increasing refractive index and density. 5. Assuming Kundt''s rule, it is deduced that the absorption spectrum of visual purple as measured directly in water solution should not coincide with its position in the rods, because of the greater density and refractive index of the rods. It is then shown that, measured by the position of the visibility curve at low intensities, this shift toward the red actually occurs, and is about 7 or 8 µµ in extent. Examination of the older data consistently confirms this difference of position between the curves representing visibility at low intensities and those representing the absorption spectrum of visual purple in water solution. 6. It is therefore held as a possible hypothesis, capable of direct, experimental verification, that the same substance—visual purple—whose absorption maximum in water solution is at 503 µµ, is dissolved in the rods where its absorption maximum is at 511 µµ, and in the cones where its maximum is at 554 µµ (or at 540 µµ, if macular absorption is taken into account, as indeed it must be).     

竹红菌光敏作用的初步探讨*

用纸片法作为筛选光敏物质及光敏作用的研究方法。结果证实了(1)竹红菌Hypo crella bambusae中含有对革兰氏阳性细菌具光敏活性的成分;(2)从中筛选到一种红色的光敏色素,属苝醌的一个新的衍生物,取名竹红菌甲素(Hypocrellin A简称“甲素”);(3)揭示了“甲素”是对可见光敏感、作用光谱较宽的光动力学(photodynamic)物质。从化合物的类型、光敏特性、治疗方法、治疗对象及其效果看,可以认为甲素是一新型的光化学疗法药物。     

竹红菌光敏作用的初步探讨

用纸片法作为筛选光敏物质及光敏作用的研究方法。结果证实了(1)竹红菌Hypo crella bambusae中含有对革兰氏阳性细菌具光敏活性的成分;(2)从中筛选到一种红色的光敏色素,属苝醌的一个新的衍生物,取名竹红菌甲素(Hypocrellin A简称“甲素”);(3)揭示了“甲素”是对可见光敏感、作用光谱较宽的光动力学(photodynamic)物质。从化合物的类型、光敏特性、治疗方法、治疗对象及其效果看,可以认为甲素是一新型的光化学疗法药物。     

STUDIES ON BIOLUMINESCENCE : XIV. THE SPECIFICITY OF LUCIFERIN AND LUCIFERASE.

Among sixteen groups of luminous forms investigated by the author, in only four (fireflies, Pholas, ostracods, and Odontosyllis) is it possible to demonstrate the luciferin-luciferase reaction. In many groups this is probably due to the small amount of these substances present in the luminescent organism or to their instability. In the medusæ and pennatulids, despite a large amount of luminescent material, luciferin and luciferase cannot be demonstrated. This does not appear to be due to the presence of luciferin and luciferase in equivalent proportion, or to their instability. In fact, one is led to the conclusion that luciferin and luciferase do not exist in these forms, but such a conclusion must be regarded as merely tentative, in view of the fundamental character of the luciferin-luciferase reaction. Luciferin of one form will not luminesce with the luciferase of another form or vice versa, unless very closely related (Cypridina and Pyrocypris). All experiments emphasize the specificity of the light producing substances of Cypridina.     

THE UPTAKE OF INORGANIC ELECTROLYTES BY THE CRAYFISH

1. Reasons are given for believing that the uptake of Na⁺, Cl^-, and NaCl by the crayfish occurs through the gills. 2. A crayfish in fresh water, with a Cl concentration of about 0.2 mEq./l., can) by active Cl absorption, compensate entirely for Cl lost in the urine. 3. The carbonic anhydrase activity of the gills is markedly higher than that of other tissues of the crayfish, but the equivalent CO₂ output of the crayfish is far in excess of the equivalent Cl absorption per unit time and weight and thus fails to warrant the supposition that Cl absorption is of respiratory importance. 4. The carbonic anhydrase activity of the soft integument of the lobster, before and after molting, and of the hypodermis of the hard-cuticled animal is almost identical and of the same order as that of other tissues of the lobster. 5. The concentration of the electrolytes was about 7.5 mEq./l.; i.e., considerably lower than in the blood of the crayfish. Cl^- can be taken up independently of the complementary cation. Na₊ can be taken up independently of the complementary anion. K⁺ and SO₄ ⁼ are not taken up at all. In pure NaCl, the Na⁺ and Cl^- are absorbed evidently largely together. Ca⁺⁺ is absorbed only in newly molted animals and in animals preparing to molt but is not absorbed by hard-cuticled animals not preparing to molt. Ca⁺⁺ is taken up independently of Cl^- in pure CaCl₂. 6. Newly molted animals absorb Ca⁺⁺ at a rate exceeding that of the absorption of other absorbable ions (Na⁺ and Cl_-) in the same equivalent concentration. 7. A crayfish utilizes the Ca⁺⁺ in fresh water in the calcification of its cuticle. Since the animal does not swallow water, the Ca⁺⁺ must enter through the exterior. Reasons are given for believing that, unlike Na⁺ and Cl^-, Ca⁺⁺ is absorbed directly from the exterior by the integument and does not enter the body through the gills. 8. During molting, only about 4 per cent of the raw ash and 2.3 per cent of the organic material of the old cuticle is resorbed.     

CONTRIBUTION TO THE THEORY OF PERMEABILITY OF MEMBRANES FOR ELECTROLYTES

On page 39, Vol. viii, No. 2, September 18, 1925, multiply the right-hand side of formula (2) by the factor See PDF for Equation. On page 44, immediately after formula (1) the text should be continued as follows: Let us suppose a membrane to be separated by two solutions of KCl of different concentrations K₁ and K₂ and these concentrations and the corresponding concentrations of K⁺ within the membrane, which are in equilibrium with the outside solutions, to be so high that the H⁺ ions may be neglected. When a small electric current flows across the system, practically the K⁺ ions alone are transferred and that in a reversible manner. Therefore the total P.D. is practically See PDF for Equation This P.D. is composed of two P.D.''s at the boundaries and the diffusion potential within the membrane. Suppose the immobility of the anions is not absolute but only relative as compared with the mobility of the cations, KCl would gradually penetrate into the membrane to equal concentration with the outside solution on either side and no boundary potential would be established. In this case the diffusion P.D. within the membrane is the only P.D., amounting to See PDF for Equation but, V being practically = 0, it would result that See PDF for Equation So the definitive result is the same as in the former case. Now cancel the printed text as far as page 48, line 13 from the top of the page, but retain Fig. 1. On page 50, line 19 from the top of the page, cancel the sentence beginning with the word But and ending with the words of the chain.     

The Luminosity Curve of the Protanomalous Fovea

Threshold spectral sensitivities (in the dark, or against bright colored backgrounds) are identical in the red-green range for both protanopes (dichromats) and protanomalous trichromatic color defectives. The latter, however, must have an additional photolabile cone pigment in the red-green range, and its presence is revealed by heterochromatic brightness matching through the spectrum (i.e. luminosity curves). The absorption spectrum of the anomalous cone pigment can be inferred from the protanomalous and protanopic luminosity curve, given reasonable assumptions as to how the different cone mechanisms pool their responses. Depending upon these assumptions, the pigment inferred is either (a) dilute solution of the normal red pigment (assumed density 1.0 for the deuteranope) or (b) similar in its absorption spectrum to the normal green pigment but shifted slightly toward the long wave end of the spectrum. Experimental attempts to choose between these alternatives have so far proved equivocal though (b) seems more likely on the basis of indirect evidence.