SIGHT 233 



eye, which upon reflection is viewed by an observer through an 

 opening in the reflector. The fundus is seen to have a reddish 

 background in which the retinal vessels are visible. 



The most important structures of the retina are the rods and 

 cones. They are closely packed on the outer surface, the rods 

 over the greater part of the retina being the more numerous. 

 They are cylindrical bodies of a transparent substance placed 

 parallel to one another and perpendicular to the surface of the 

 eyeball. The cones, which are modifications of the rods, are 

 very similar to the latter, but do not reach the same level. 

 These structures are connected through intermediate neurons 

 with the fibers of the optic nerve. Where this nerve enters the 

 retina, a little to the inner side of the most posterior point of the 

 eyeball, there are no rods or cones, so that an image focussed 

 at this point will be followed by no perception. This point is 

 called the blind spot. 



At the exact centre of the retina that is, the most posterior 

 point of the eye there is a small yellow area (macula luted) 

 with a central depression (fovea centralis). Here none of the 

 fibers of the optic nerve are to be found, but a great increase 

 in the number of cones as well as an increase in their size. If 

 the object looked at is focussed directly upon the fovea cen- 

 tralis, the image is seen with greatest clearness. In e very-day 

 life images are received upon the macula lutea, and rays of 

 light entering the eye at an angle are focussed on some other 

 part of the retina, and are not defined so clearly. 



A retina which has been protected from the light for a time 

 has a purplish-red color, due to a coloring matter termed 

 visual purple. This is confined to the outer portions of the rods, 

 and does not reach the cones. It is bleached by light, but 

 restored by the pigment epithelium. The retina of a rabbit 

 may be impressed with an image focussed upon it and then 

 treated with a 4 per cent, solution of alum, which "fixes" it 

 and prevents the restoration of the visual purple. Such a 

 picture is called an optogram. 



Vibrations of the ether form the normal stimulus for the 

 retina, the rods and cones of which form, perhaps, the only 

 structures of man that can supply the necessary conditions 

 for the transformation of radiant energy into the energy of a 

 nerve impulse. The ether vibrations vary widely in their 



