68 



NA TURh 



[November 21, 1895 



optic nerve. Light passes across these, but it does not excite 

 the nerves in passing through them. The nerves are trans- 

 parent, and the light produces no effect upon them directly. If 

 it did, your whole field of view would be confused, because it is 

 known that when a nerve is excited the sensation is referred to 

 a particular part no matter where the nerve may be affected. 

 Suppose you could isolate, say in the thigh, a particular nerve lead- 

 ing to the great toe, and pinch it without hurting its neighbours, 

 you would feel the pinch not where the nerve is pinched, but in 

 the great toe. So, here, if these nerve-fibres were excited by 

 the passage of light through them, then the sensation corre- 

 sponding to the excitement of a particular nerve -fibre, which 

 would be that of a definite point in the field of view, would be 

 excited by an external luminous point lying anywhere in the 

 curve in which the surface generated by a straight line passing 

 through the optical centre and intersecting the fibre in question 

 would cut what we may call the celestial sphere, and the corre- 

 spondence between the subjective points in the field of view and 

 objective external points would be lost. And the fact that the 

 visual nerves are not affected by light which passes across 

 them is further shown by the well-known experiment of the 

 blind spot, where the optic axis passes out of the eye-ball, not 

 in the axis of vision but to one side, towards the nose, so that 

 an object whose image falls on the blind spot of one eye is seen 

 by means of the other. 



But now comes a question, and here we enter on uncertain 

 and debated ground — How is it that the nerves are stimulated 

 by the light at all ? 



We have reason to believe that these rods and cones form 

 the means by which the light, acting on them, causes the 

 stimulation of the nerve. As I have said, they consist of two 

 elements, an inner and outer ; the outer from the centre of the 

 eye, i.e. the inner as regards the body, being of that remark- 

 able structure which I have described. It has been questioned 

 which of these two elements it is that you are to regard as the 

 percipient organ. I do not know that physiologists have 

 decided that question. I have looked into a paper of Max 

 Schultze's — in fact I have it on the table — and he inclines to the 

 opinion that it is the outer element. Now is there anything in 

 the outer element which can conceivably form a means of 

 stimulation of the nerve, when that element is acted on by 

 light? 



I have spoken of the way in which it is composed of laminae 

 which come to pieces when dissected, after a certain amount of 

 maceration. I do not know whether it may not be rash to say 

 what I am about to say, because I do not know that physio- 

 logists have suggested it — it is merely an idea which occurred to 

 myself, so you must take it for what it is worth. I was reading 

 an account of the electric organ of electrical fishes, such as the 

 torpedo. It is a very remarkable organ, occupying a consider- 

 able space in these fishes. It has a columnar structure, and the 

 column again consists of laminos placed one over the other. It 

 has a structure which may roughly be compared to that of the 

 basaltic columns in the Giant's Causeway, only here you must 

 think of laminae as more numerous and not having that curved 

 surface shown in the Giant's Causeway. Now nobody questions 

 that somehow or other this is an organ by means of which these 

 fishes are enabled to give a shock, and the idea, of course, is 

 suggested, are not these laminte like the plates of a battery ? 

 Is not one of these columns, roughly speaking, something like a 

 galvanic battery ? But how the battery is charged and dis- 

 charged we do not know. In this case it depends, no doubt, on 

 the will of the animal as to what he does, and nobody knows 

 how he brings that about. 



Now it strikes me that there is a remarkable apparent analogy 

 between the outer member of the rods and cones, and these 

 columns in electrical fishes. This gives rise to the suspicion 

 that possibly these outer members may act the part of a micro- 

 scopic battery, being charged somehow or other. But how 

 are they to be charged ? Well, before I go on to enter into any 

 speculation on that I may mention that some years ago Prof. 

 Dewar and Mr. McKendrick made some remarkable experi- 

 ments, the results of which are given in a paper published in the 

 Transactions of the Royal Society of Edinburgh. When an eye 

 is dissected out, and the cornea is connected through a wire with 

 non-polarising electrodes to the middle of the section of the optic 

 nerve, the wire being led through a delicate galvanometer, it is 

 found that there is a certain amount of electric current passing. 

 Now it was found that when the eye (having been in darkness) 

 was allowed to have light shining upon it, there was a change in 



NO. 1360, VOL. 53] 



this current, and a change again when the light was cut off. It 

 is true that the total change was only a small fraction of the 

 whole ; but still that there should be any change at all produced 

 by the action of light is a remarkable thing. It looks very much 

 as if the stimulation of the nerve had something or other to do 

 with the production of electric currents ; but those, if they are 

 produced, we must suppose to be produced in some way by the 

 action of light. How may we imagine light to act so as to 

 produce them ? It has been discovered that in the layer of pig- 

 ment cells in the retina there is a substance, called visual purple, 

 of a purple colour, which is acted on by light, and is made first 

 yellow and then nearly colourless. We have thus a substance 

 that is capable of being acted upon by light, as very many sub- 

 stances are. I do not say that it is by any means proved that 

 that is the substance, or even that there is any substance, which 

 is acted upon by light in the way demanded ; yet it seems very 

 probable that the change produced by the action of light, 

 whether it be on visual purple, or some other substance associated 

 with it, may give rise to something which may, so to speak, 

 charge this microscopic battery and stimulate the nerve-fibre 

 which is attached to it. We know the rate of the vibrations of 

 light of various kinds ; and the rapidity of vibrations is so 

 enormous, ranging about 400 millions of millions of vibrations in 

 a second, that we can hardly imagine that the organism of our 

 bodies is calculated to be set in vibration in a corresponding 

 period. In that respect the sense of sight differs notably from the 

 sense of hearing. In hearing the tympanum of the ear is thrown 

 into vibration, and the vibrations are not so enormous in 

 number in such a time as one second but that the corre- 

 sponding nerves may actually be mechanically agitated, and 

 thereby in some way stimulated. We can hardly imagine that 

 the visual nerves are acted upon in this sort of way directly by 

 the luminous vibrations, but they may be indirectly. Here, 

 again, I may throw out a possible conjecture, though I am less 

 disposed to receive it myself than that which I have just men- 

 tioned. We know there are substances which when acted upon 

 by light continue to shine in the dark. In some cases the action 

 ceases almost instantly after the exciting light is cut off ; for in- 

 stance, a solution of the salts of quinine, where the rapidity of 

 cessation of the effect is amply sufficient to tally with the rapidity 

 of cessation of visual sensation when light is cut off. 



There are various other matters connected with the perception 

 of light which are of great importance to our well-being and to 

 our enjoyment which I have not ventured to touch upon at all. 

 It would take a great deal too long to go into two which I will 

 only just mention. One is the provision in the two eyes, and in 

 the muscles which move them, which enables us to obtain single 

 vision notwithstanding that the two eyes are at work. Nothing 

 is easier than to obtain double vision in which the images seen 

 by means of two eyes occupy different positions in the field of 

 view. There are very remarkable contrivances for bringing 

 about singleness of vision in the habitual use of both eyes. 



Then, again, we do not see light merely as light, but we see a 

 great variety of colour. We can distinguish one light from another 

 light by its colour, and not by its intensity only. It would take 

 me a great deal too long to give you any idea of what is 

 known (which after all is not much) as to the way in which that 

 is effected. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



Cambridge. — The Woodwardian Museum has been greatly 

 enriched by the generosity of Prof. T. Wiltshire, Treasurer of 

 the Geological Society and Secretary of the Palseontographical 

 Society, who has presented to the University a large collection 

 of Cretaceous fossils. The collection includes nearly all the 

 known British species, as well as many not yet described. This 

 addition will probably make the Woodwardian collection of 

 Cretaceous fossils the finest in the country. The thanks of the 

 University have been voted to Prof. Wiltshire, who is himself a 

 Cambridge graduate. 



Sir William Turner, F.R.S., of Edinburgh, has been ap- 

 pointed an Elector to the chair of Anatomy, and Lord Walsing- 

 ham, F.R.S., an Elector to the chair of Zoology and Compara- 

 tive Anatomy, in place of the late Right Hon. T. H. Huxley. 



The Vice-Chancellor, Mr. C. Smith, Master of Sidney Sussex 

 College, is suffering from the shock of a fall from his bicycle last 

 week, and will probably be unable to resume his duties for some 



