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KNOWLEDGE & SCIENTIFIC NEWS. 



[December, 1906. 



Having examined the nature of light and vision thus 

 tar, tile question w Iiich naturally suggests itself is, how 

 is the energy of light eonverted into a nerve impulse, 

 and if, as seems probable, there arc only three primary 

 eolour sensations, by what means are these severally 

 differentiated? 



The retina consists of several distinct layers of living 

 protoplasmic cells, the most remarkable of which are 

 the layer of rods and cones, which are found on the 

 surface, and which, consequently, are the first to re- 

 ceive the impressions of incident rays of luminiferous 

 ether. What happens in these cells under its influence? 

 We know by examination of other tissues that proto- 

 plasm has the power of forming in its life processes 

 certain bodies called ferments, which, under certain 

 conditions and in the presence of favourable surround- 

 ings, produce chemical changes, either katabolic (de- 

 structive) or analxilic (constructive), without themselves 

 being in any way affected. For instance, the gastric 

 cells produce a ferment called pepsin, which is able to 

 convert the proteids of food into peptones ; so also the 

 pancreatic cells possess a ferment called amylopsin, 

 which is able to convert starches into sugar, with the 

 evolution of various forms of energy, chemical, 

 nervous, &c. Is it not possible, too, in case of the 

 retinal cells, that the process by which the energy of 

 light is converted into nervous energy may be a process 

 of fermentation, and that the ions of luminiferous ether, 

 acting on the ions set free by a ferment body present in 

 these protoplasmic cells, may produce katabolic, and 

 possibly anabolic, changes, which, liberating electro- 

 vital force and nerve stimulation, arc conducted by the 

 piliments of the optic nerve to the visual centres in the 

 brain, to be there interpreted by the consciousness as 

 sensations of light and colour? 



That light will produce these changes in the retinal 

 cells is well illustrated by Waller's researches, and that 

 the presence of ferment bodies in protoplasmic cells 

 may, through ionic action, give rise to nerve force is 

 supported by Dr. Allchin in a lecture given by him on 

 " Nutrition and Malnutrition," reported in the Clinical 

 Journal, April, 1905. 



The former experiments are described in Halli- 

 burton's "Handbook of Physiology" as follows : — "Tlie 

 excised eyeball of a frog is led off by non-polarisable 

 electrodes to a galvanometer. One electrode is placed 

 on the front, the other on the back of the eye. A 

 current of rest (demarcation current) is observed pass- 

 ing through the eyeball from front to back. When 

 light falls on the eye this current is increased; on 

 shutting off the light there is a momentary further in- 

 crease, and then the current slowly returns back to its 

 previous condition. Waller explains this by supposing 

 that anabolic changes in the eye predominate during 

 stimulation by light. With the onset of darkness, the 

 katabolic changes cease at once, and the anabolic more 

 slowly; hence a further positive variation. If the eye- 

 ball has been excised the day before the observations 

 are made, or has been fatigued or injured, light pro- 

 duces principally katabolic changes, as evidenced by a 

 negative variation. A slight positive effect follow-s 

 when the light is shut off." 



On the question of ferment bodies producing nerve 

 energy, &c.. Dr. Allchin expresses himself as follows : — 

 " The vital acti\-ities of the living cells would seem to 

 consist essentially in the formation of ferment bodies 

 which alone, or in combination, effect those integrations 

 and disintegrations which liberate chemical energ^y, and 

 that this by transformation produces muscular work, 

 n<'rve force, and secretory function, the fundamental 

 manifestations of life. That these enzymes do bring 



about these changes in such conditions of temperature 

 and alkalinity or acidity as obtain in the body appears 

 to be certain, and as an explanation of the activity of the 

 bioplasm, which elaborates these bodies, there is postu- 

 lated an ionic action on the part of the cell contents, 

 and their surrounding medium whereby charges of 

 electricity of variable strength and character are 

 brought into conflict, and that from the play of ions the 

 manifestations of vitality result." Whether visual 

 purple is of the nature of a ferment, as seems to be 

 suggested by Hering's theory of colour vision, is not 

 at present known, neither has its presence been demon- 

 strated in the human retina as far as I am aw^are. 



That ionic action should be produced by light in the 

 presence of a ferment contained in the retinal cells 

 would not be incompatible with the Young-Helmholtz 

 theory of colour vision depending on three primary 

 colour sensations, red, gieen, and violet, if we suppose 

 that there are present in these cells three ferments 

 capable of specially responding to each of these radia- 

 tions (or one ferment even having the property of three 

 separate reactions in an ascending scale of katabolism). 

 It is important to note the position in the spectrum of 

 these three radiations. On the extreme left are those 

 which give rise to the sensation-red, of comparative 

 long wave-length; that is, those which act least power- 

 fully on the photographic plate; in other words, whose 

 actinic or disintegrating powers are least powerful of 

 the luminiferous rays; in the middle are those which 

 give rise to the sensation of green, where actinic action 

 occupies an intermediate position; whilst at the extreme 

 right are those of shortest wave-length, which give rise 

 to the sensation of violet. These are the so-called 

 " actinic waves," whose actinic action is greatest, and 

 which act most powerfully on the photographic plate. 



Therefore, granting that in accordance with the 

 Young-Helmholtz theory there are in the retina rods 

 and cones which answer to each of these three primary 

 colour sensations, and bearing in mind the above facts 

 that the radiations producing them respectively occupy 

 three fixed points in the spectrum, left, middle, and 

 right, in an ascending scale of actinism (power to pro- 

 duce chemical cliange), I would suggest that an ionic 

 action is induced by these radiations in association with 

 three distinct ferments present severally in the rods 

 and cones specialised to receive them, each ferment be- 

 ing specially capable of producing katabolic changes 

 under the influence of the particular radiation con- 

 cerned, and out of the energy thus liberated three corre- 

 sponding degrees of nerve stimulation arise to affect the 

 nerve cells in the deepest layer of the retina, which, on 

 being transmitted by the nerve fibres to the nerve cells 

 in the visual centres of the brain, are interpreted by the 

 consciousness as the above colour sensations. The 

 various other shades and colour effects seen in Nature 

 are probably due, as the Young-Helmholtz theory 

 teaches, to the different degrees of stimulation these 

 three-colour terminals receive. Thus, if a large num- 

 ber of rods or cones, responding to the radiations to 

 the extreme left of the spectrum, are brought under the 

 influence of these rays, and those which respond to 

 green and violet are hardly affected by their correspond- 

 ing radiations, the sensation would be red. If, how- 

 ever, orange is the colour sensation produced, then it 

 will be owing to the red terminals or rods and cones 

 corresponding to red that are considerably influenced, 

 the green rather more, and the violet only slightly so, 

 8cc. In bringing this paper to a close I must acknow- 

 ledge my indebtedness to Watson's " Text-book of 

 Physics," Ganot's " Popular Natural Philosophy," and 

 Halliburton's " Handbook of Physiologfy. " 



