THE ETE AS A SENSORY APPARATUS. 



533 



the retina (which is 15mm.) be F, then the distance, on 

 the paper, of the cross from the circle will be 

 to the distance of y from o as/ is to F. Meas- 

 urements made in this way show that the circle 

 disappears when its image is thrown on the 

 entry of the optic nerve, which lies to the nasal 

 side of the yellow spot. 



2. The above experiment having shown that 

 light does not act directly on the optic nerve- 

 fibres any more than it does on any other nerve- 

 fibres, we have next to see in what part of the 

 retina those changes do first occur which form 

 the link between light and nervous impulses. 

 They occur in the outer part of the retina, in 

 the rods and cones. This is proved by what is 

 called Purkinje's experiment. Take a candle in- 

 to a dark room and look at a surface not covered 

 with any special pattern, say a whitewashed wall 

 or a plain window-shade. Hold the candle to the side of one 

 eye and close to it, but so far back that no light enters the 

 pupil from it; that is so far back that the flame just can- 

 not be seen, but so that a strong light is thrown on the white 

 of the eye as far back as possible. Then move the candle a 

 little to and fro. The surface looked at will appear luminous 

 with reddish-yellow light, and on it will be seen dark branch- 

 ing lines which are the shadows of the retinal vessels. Now 

 in order that these shadows may be seen the parts on which 

 the light acts must be behind the vessels, that is in the outer 

 layers of the retina since the blood vessels lie in its inner 

 strata. The experiment may be more satisfactorily performed 

 by getting another person to focus with a lens the light of 

 the candle as a bright spot as far back as possible on the white 

 of the observer's eye. 



If the light be kept steady the vascular shadows soon dis- 

 appear ; in order to continue to see them the candle must be 

 kept moving. The explanation of this fact may readily be 

 made clear by fixing the eyes for ten or fifteen seconds on the 

 dot of an"i" somewhere about the middle of this page : at 

 first the distinction between the slightly luminous black 

 letters and the highly luminous white page is very obvious ; 

 in other words, the different sensations arising from the 

 strongly and the feebly excited areas of the retina. But if 



