2g6 



NATURE 



[June 13, 1918 



passing along the auditory nerve when a sound is 

 heard. 



In addition to the rod and cone layer, the retina of 

 the vertebrate -contains several layers of nerve-cells. 

 These do not really belong to the receptor organ 

 itself, but are probably of the nature of relays. In 

 the cuttle-fish they form a separate mass, outside the 

 eye itself. 



We know that the cones are sensitive to light, 

 because they are the only elements present in the 

 most sensitive ^spot of the retina, the fovea centralis. 

 That of the rods is more disputed, but their nervous 

 connections are similar to those of the cones, so that it 

 is difficult to believe that they are not also receptors 

 for light. 



The only satisfactory explanation of the mode of 

 stimulation of the retina by light is that it is through 

 the intermediation of a chemical reaction brought 

 about by radiant energy of a limited series of wave- 

 lengths. As yet only one substance sensitive to light 

 has been discovered in this situation. This is usually 

 known as "visual purple," but its actual colour would 

 be described by most people as deep rose red. 



Remembering, in the first place, Grotthus's law, that 

 light can produce an effect only in proportion as it is 

 absorbed, it is of importance to investigate the absor- 

 bent properties of visual purple as regards that part 

 of the spectrum which is visible to us. This has been 

 done by Victor Henri and Larguier des Bancels.^ 

 They determined the absorption of light by solutions 

 of visual purple, compared it with the degree of 

 chemical action, and also with the minimal amount 

 of light energy required to excite a just perceptible 

 sensation from the peripheral parts of the retina. The 

 determinations were made throughout the visible spec- 

 trum, and the three curves show a remarkable agree- 

 ment. These facts show that visual purple is at 

 least the most important photo-chemically sensitive 

 component of the retina, if not the only one. We 

 note also that there are no absorption bands in its 

 spectrum, so that there is no difficulty with respect 

 to colour vision. An interesting fact conies out from 

 the curve of sensibility of the retina compared with 

 the energy of the light acting. At that particular 

 frequency of vibration corresponding with the yellow- 

 green, the threshold of stimulus coincides with the 

 energy quantum of Planck for that rate of vibration. 

 In other words, the retina is sensitive to as small an 

 incidence of energy as it is possible for it to receive. 



Very little is known as to the chemical nature of 

 visual purple. It exists in colloidal solution in a liquid 

 in which the rods and cones are immersed. Although 

 it is not produced in the fovea itself. Dr. Edridge- 

 Green has found evidence that it flows in from sur- 

 rounding parts. 



The best information to be obtained of the course 

 of the photo-chemical reaction is obtained from records 

 of the electrical changes which occur on the incidence 

 of light. The simpler curve given by the eye of the 

 cuttle-fish is to be regarded as indicating the essential 

 part, of the phenomenon; the more complex form of 

 the vertebrate curve is probably due to the presence 

 of the extraneous nerve-cells. The chief points to 

 be noticed are the following : — 



First, the curve gradually falls, the stimulus merely 

 disappears, on the advent of darkness. There is no in- 

 dication of a stimulus of any kind produced by dark- 

 ness. This is contrary to the well-known theory of 

 Hering, according to which the reaction of restoration, 

 occurring when the light ceases, is associated with a 

 positive sensation of darkness. This point of view 

 had been applied to physiological phenomena in 

 general, but is now prdctically given up. 



* V. Henri et J. Larguier des Bancels, Joum. de Physiol, et Pathol. 

 ol. xiii., pp. 841-56 (igii). 



NO. 2537, VOL. lOl] 



Secondly, the curve, after it has attained its maxi- 

 mum, remains constant while the illumination lasts. 



Thirdly, the reaction does not attain its full intensity 

 suddenly, nor do the products disappear suddenly. In 

 other words, the sensation does not appear at once, nor 

 does it immediately disappear when the stimulus ceases. 

 This is the obvious explanation of the absence of 

 flicker when the alternations of light and darkness 

 are sufficiently rapid. Further, as would be expected 

 from a chemical reaction, the greater its magnitude, 

 the longer it requires for the products to recombine 

 or otherwise disappear. Incidentally, the form of the 

 curve differs somewhat for different colours. 



Fourthly, there is a short latent period between the 

 time of incidence of light and the electrical effect. If 

 this is not counterbalanced by a similar period after 

 the illumination ceases, it would result in some devia- 

 tion from Talbot's law in its physiological aspect, such 

 as has been described by Parker and Patten. The 

 latent period reminds us of the " phota-chemical induc- 

 tion" of Bunsen and,Roscoe. 



There is reason to believe that the maximum sensi- 

 bility of the fovea is not when it is the only part of 

 the retina illuminated, but when there is simultaneously 

 a weak illumination- of the surrounding parts. This 

 seems to be connected with the production and move- 

 ment of the visual purple. If it be the fact, its im- 

 portance in observations with the microscope, the 

 polarimeter, and other optical instruments is obvious. 



The explanation of positive and negative after- 

 images is fairly plain — the former by the products of 

 photo-chemical change not disagpearing at once, the 

 latter by temporary exhaustion of the visual purple. 

 Edridge-Green * has shown that the situation and 

 shape of the positive after-image can be altered by 

 jerking the head, showing that the chemical change 

 is located in the liquid surrounding th? rods and cones. 

 Hence these structures must .be affected secondarily. 

 The negative after-image is fixed, indicating a situa- 

 tion in the more solid parts of the receptive 

 mechanism. 



The adaptation of the retina to various degrees of 

 illumination — an important fact in daily life — is probably 

 due to a change in the position of the pseudo-equili- 

 brium which results from the fact that the products 

 of a reversible photo-chemical reaction are continually 

 recombining during the illumination itself. The cir- 

 cumstance that this adaptation is not a very rapid 

 process is of importance in relation to the practical 

 aspect of artificial illumination, and shows that sudden 

 changes in the lighting of neighbouring objects are 

 not desirable. The suggestion that the ratio of bright- 

 ness of objects to which the eye turns should not 

 exceed i : loo seems a reasonable one. 



The problem of " glare " is also connected, although 

 the fact of the unpleasant and injurious effect of 

 powerful local stimulation of the retina has also to 

 be taken into account. But, like so many of the 

 practical problems we meet with, it requires much 

 more investigation, and the co-operation of the physio- 

 logist, the illuminating engineer, the oculist, and the 

 factory inspector is much to be desired. 



The effect of lateral illumination brings up the ques- 

 tion of the function of the rods as distinct from that 

 of the cones, as do also vision under weak illumination 

 and that known as "night-blindness." The first 

 problem has been very able treated by Dr. Herbert 

 Parsons in a Home Office report, while the enormous 

 range of intensity of the light perceived has been em- 

 phasised by Mr. Trotter. This degree of " adaptation " 

 is much greater, than could be accounted for by vary- 

 ing apertures of the pupil. 



The question of the colour of the light in relation 



•• Journ. 0/ Physiol., vol. xlv., p. 70. - 



