260 SIR G. STOKES^ BART., ON THE PERCEPTION OP COLOUR. 



(lenoraiiiated rods and cones. The number of these in the 

 eye is enormous. I have here a drawing* giving the facts 

 to which I am now pointing in regard to the retina. You 

 look on the eye from behind so that you see the ends of 

 those rods and cones. Those rods and cones are richly 

 provided with excessively delicate minute nerve fibres, and 

 there is little doabt that somehow or other the ends or end 

 portions of those nerve fibres are excited by the influence 

 of light, and convey the stimulus on through the set ot 

 nerves lining the retina further in front, where they are 

 crossed by the light without being affected thereby, and 

 at last unite in a bundle forming the optic nerve, and pass 

 into the brain. 



Now it has been found that in the central part of the 

 retina of the human eye, where vision is most acute, and 

 where there are cones only, without rods, the distance 

 between consecutive cones is about '003 of a millimetre — an 

 excessively small quantity — and we can easily calculate hi- 

 dependently the approximate distance on the retina of the 

 images of two visible points which can just be seen as two, 

 supposing, of course, that, in the first instance, we have 

 determined experimentally the angular distance of those 

 visible points. It turns out that the distance of the images 

 corresponds very closely indeed with the distance apart of 

 the cones in the bacillary layer of the retina, so that ap- 

 parently the stimulation of one of those gives us the per- 

 ception of a single point in the field of view, the apparent 

 position of which varies with the position in the retina of 

 the particular cone on which the image falls. If we view a 

 star we have the sensation of a point of white light in 

 a particular direction. If we hold a red or green or blue 

 glass before the e^^e, we have the sensation of a point of red 

 or green or blue light in the same direction. On the theory 

 of three primary colour sensations, whatever those may be, we 

 niust infer that the stimulation of the same cone is capable of 

 giving rise to all three of the primary colour sensations, but 

 that the difference of colour sensation does not entail a differ- 

 ence of apparent direction. Can we form any idea as to how 

 these conditions may be fulfilled 1 



Dr. Young's idea was that there are three kinds of nerve 



■* Referring to one of the plates in a j^aper by Max Schultze in the 

 2nd volume of the Archiv fiir Microscopische Anatomic. 



