THE EFFECT OF EXPOSURE PERIOD AND TEMPERATURE ON THE PHOTOSENSORY PROCESS IN CIONA

1. Experiments are presented which show that the latent period in the photosensory response of Ciona is inversely proportional to the duration of the exposure period to light. From this it is found that the velocity of the chemical reaction which determines the latent period is directly proportional to the concentration of photochemical products formed during the exposure period. This is interpreted as showing that the two processes form a coupled photochemical reaction, of which the secondary reaction proceeds only in the presence of products from the primary reaction. This coupling may be a catalysis or a direct chemical relation. 2. Further experiments show that the relation between temperature and the latent period is accurately described by the Arrhenius equation in which µ = 16,200. The precise numerical value of µ tentatively identifies the latent period process as an oxidation reaction which is catalyzed by iron. 3. The photocatalytic properties of certain iron compounds are used as a model for the coupled photochemical reaction suggested for the photosensory mechanism of Ciona and Mya

THE PHOTOCHEMICAL NATURE OF THE PHOTOSENSORY PROCESS

1. In order to produce a response in Mya, the minimum amount of light energy required is 5.62 meter candle seconds. This energy follows the Bunsen-Roscoe law for the relation between intensity and time of exposure. 2. The necessary minimum amount of energy varies but little with the temperature; the temperature coefficient for 10°C. is 1.06. 3. In view of these facts it is concluded that the initial action of the light is photochemical in nature. This substantiates the hypothesis previously suggested to account for the mechanism of photoreception. 4. The constant energy requirement for stimulation of Mya shows that the traditional division of animals into those which respond to a constant source of light and those which respond to a rapidly augmented light is without any fundamental significance for sensory physiology

PHOTOCHEMISTRY OF VISUAL PURPLE : II. THE EFFECT OF TEMPERATURE ON THE BLEACHING OF VISUAL PURPLE BY LIGHT.

The temperature coefficient of the bleaching of visual purple by light is 1.00 over a range of 30 degrees. This indicates that the monomolecular course of the reaction represents a real chemical process, as opposed to a possible diffusion process, and that the reaction is probably simple in nature

THE EFFECT OF TEMPERATURE ON THE LATENT PERIOD IN THE PHOTIC RESPONSE OF MYA ARENARIA

1. The effect of temperature on the reaction time of Mya to light is mainly confined to the latent period. The sensitization period, representing a photochemical process, is changed comparatively little. 2. The relation between the latent period and the temperature is adequately expressed by the Arrhenius equation, for temperatures below 21°C. Above this temperature, the latent period becomes increasingly longer than is required by the Arrhenius formula when µ = 19,680. 3. These deviations, occurring above the highest environmental temperature of Mya, are explained on the assumption that the principal product formed during the latent period is inactivated by heat. 4. Calculation of the velocity of the hypothetical inactivation reaction at different temperatures shows that it also follows the Arrhenius rule when µ = 48,500. This value of µ corresponds to those generally found for spontaneous inactivations and destructions

INTENSITY DISCRIMINATION AND THE STATIONARY STATE

1. A method of experimentation is described which enables one to record objectively and quantitatively the discrimination by Mya between two intensities of illumination to which it is successively exposed. The indicator for this discrimination is a response at a given reaction time. 2. From the data so obtained it is found that the difference, ΔI, between the two intensities bears no constant relation to the initial intensity, I. Instead, the ratio See PDF for Equation varies in a consistent manner with I. As the latter increases, the ratio decreases to a certain point, after which it increases. 3. The data are analyzed in terms of the photochemical mechanism previously proposed for the sensitivity of Mya to light. It is shown that for the animal to discriminate by means of a given reaction between one intensity and another, the transition from one to the other must be accompanied by the decomposition of a constant amount of photosensitive substance in the sense organ. 4. A mathematical treatment of the behavior of the photochemical mechanism shows not only that the ratio See PDF for Equation cannot be constant as required by the Weber-Fechner law, but that it must vary in the way in which it does. The behavior of Mya under these conditions, therefore, supports the validity of the hypothetical physicochemical mechanism suggested for its sensitivity

THE RELATION BETWEEN THE WAVE-LENGTH OF LIGHT AND ITS EFFECT ON THE PHOTOSENSORY PROCESS

1. Following the description of a simple method of securing high intensities of monochromatic illumination, it is shown that the most effective portion of the spectrum for the stimulation of Mya is near λ = 500 µµ. 2. The quantitative data secured is interpreted in terms of certain photochemical findings, and as a result the absorption spectrum of the photosensitive substance of Mya is tentatively mapped out

THE NATURE OF FOVEAL DARK ADAPTATION

1. After a discussion of the sources of error involved in the study of dark adaptation, an apparatus and a procedure are described which avoid these errors. The method includes a control of the initial light adaptation, a record of the exact beginning of dark adaptation, and an accurate means of measuring the threshold of the fovea after different intervals in the dark. 2. The results show that dark adaptation of the eye as measured by foveal vision proceeds at a very precipitous rate during the first few seconds, that most of the adaptation takes place during the first 30 seconds, and that the process practically ceases after 10 minutes. These findings explain much of the irregularity of the older data. 3. The changes which correspond to those in the fovea alone are secured by correcting the above results in terms of the movements of the pupil during dark adaptation. 4. On the assumption that the photochemical effect of the light is a linear function of the intensity, it is shown that the dark adaptation of the fovea itself follows the course of a bimolecular reaction. This is interpreted to mean that there are two photolytic products in the fovea; that they are disappearing because they are recombining to form anew the photosensitive substance of the fovea; and that the concentration of these products of photolysis in the sense cell must be increased by a definite fraction in order to produce a visual effect. 5. It is then suggested that the basis of the initial event in foveal light perception is some mechanism that involves a reversible photochemical reaction of which the "dark" reaction is bimolecular. Dark adaptation follows the "dark" reaction; sensory equilibrium is represented by the stationary state; and light adaptation by the shifting of the stationary state to a fresh point of equilibrium toward the "dark" side of the reaction

PHOTOCHEMISTRY OF VISUAL PURPLE : III. THE RELATION BETWEEN THE INTENSITY OF LIGHT AND THE RATE OF BLEACHING OF VISUAL PURPLE.

It is shown that the velocity of bleaching of visual purple by light, under comparable conditions of concentration, volume, and surface exposed, is directly proportional to the intensity

THE RELATION BETWEEN VISUAL ACUITY AND ILLUMINATION

1. Visual acuity varies in a definite manner with the illumination. At low intensities visual acuity increases slowly in proportion to log I; at higher intensities it increases nearly ten times more rapidly in relation to log I; at the highest illuminations it remains constant regardless of the changes in log I. 2. These variations in visual acuity measure the variations in the resolving power of the retina. The retina is a surface composed of discrete rods and cones. Therefore its resolving power depends on the number of elements present in a unit area. The changes in visual acuity then presuppose that the number of elements in the retina is variable. This cannot be true anatomically; therefore it must be assumed functionally. 3. To explain on such a basis the variations of visual acuity, it is postulated that the thresholds of the cones and of the rods are distributed in relation to the illumination in a statistical manner similar to that of other populations. In addition the rods as a whole have thresholds lower than the cones. Then at low intensities the increase in visual acuity depends on the augmentation of the functional rod population which accompanies intensity increase; and at higher intensities the increase in visual acuity depends on the augmentation of the functional cone population. The number of cones per unit foveal area is much greater than the number of rods per unit peripheral area, which accounts for the relative rates of increase of rod and cone visual acuity with intensity. At the highest illuminations all the cones are functional and no increase in visual acuity is possible. 4. If this division into rod visual acuity and cone visual acuity is correct, a completely color-blind person should have only rod visual acuity. It is shown by a study of the data of two such individuals that this is true. 5. The rod and cone threshold distribution has been presented as a purely statistical assumption. It can be shown, however, that it is really a necessary consequence of a photochemical system which has already been used to describe other properties of vision. This system consists of a photosensitive material in reversible relation with its precursors which are its products of decomposition as well. 6. On the basis of these and other data it is shown that a minimal retinal area in the fovea, which can mediate all the steps in such functions as visual acuity, intensity discrimination, and color vision, contains about 540 cones. Certain suggestions with regard to a quantitative mechanism for color vision are then correlated with these findings, and are shown to be in harmony with accurately known phenomena in related fields of physiology

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