224 STUDIES IN ELECTRO-PHYSIOLOGY: 



taking place always in planes at right angles to the 

 direction of the ray, but in all directions in those planes. 

 That is, if the ray travels along the axle of a wheel, the 

 vibrations composing it are all in the plane of the wheel, 

 but are executed along any or all of the spokes." (Gordon's 

 Electricity and Magnetism.) 



Rays of light, entering at the lens, would, if the lens 

 were a fixed object, approximate to the axle, and the rods 

 and cones to the spokes of the wheel. But the lens is not 

 a fixed object, as in a camera. It not only receives rays 

 of light from above, below, and each side, but continually 

 shifts its angle of reception of such rays by movement of 

 the eye. 



Fig. 120. 



Diagram of a section through the (right) human eye passing horizontally 

 nearly through the middle, a, b, equator ; y, optic axis. (After Sch&fer.) 



The pigmented cells of the outer or choroidal surface 

 are not shown in Fig. 121, but are illustrated by Schultze 

 in a diagrammatic section of the human retina (Fig. 122). 



