FUNCTION OF RODS AND CONES. 



509 



vertically in front of the face, and moving it to and fro. 

 It will be found that at about 25 centimeters (10 inches) 

 off the white circle disappears; but when the page is nearer 

 or farther, it is seen. During the experiment the gaze must 

 be kept fixed on the cross. There is thus in the field of 

 Tision &Jblind spot, and it is easy to show by measurement 

 that it lies where the optic nerve enters. 



When the right eye is fixed on the cross, it is so directed 

 that rays from this fall on the yellow spot, y, Fig. 138. 

 The rays from the circle then cross the visual axis at the 

 nodal point, n, and meet the retina at o. 

 If the distance of the nodal point of the 

 -eye from the paper be /, and from the 

 retina (which is 15 mm.) be F, then the dis- 

 tance, on the paper, of the cross from the 

 circle will be to the distance of y from o as 

 / is to F. Measurements 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 (p. 487). 



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 

 oandle into a dark room and look at a surface not covered 

 ivith any special pattern, say a whitewashed wall or a plain 

 -window-shade. Hold the candle to the side of one eye and 

 dose to it, but so far back that no light enters the pupil 

 ifrom it; that is so far back that the flame just cannot 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 



FIG. 138. 



