THE PERCEPTION OF LIGHT. 19 



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 

 m darkness) Avas allowed to have light shining upon it, there 

 was a change in 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 remark- 

 able thing. It looks very much as if the stimulation of the 

 nerve had something or other to do with the production ot 

 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 pigment 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 

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

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

 Avhich 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 

 v/hich 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 corres- 

 ponding 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 con- 

 jecture, though 1 am less disposed to receive it myself than 

 that which I have just mentioned. We know there are 



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