ELEMENTARY EXPERIMENTAL PHYSIOLOGY 



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screen which is used as a receiving surface for the refracted rays, and 

 an opaque screen with a central hole. 



The box is first filled with water, and in order to make rays of light 

 the clearer, a few drops of some fluorescent solution (e.g. eosin) are 

 added to the water. An external luminous object is then arranged. 

 This may be conveniently done by placing a metal plate, in which a 

 vertical arrow has been stencilled out, in front of a good source of light, 

 such as the naked arc light of an electric projecting lantern, with the 

 condenser and focussing lens removed. This stencilled plate is placed 

 four or five feet from the front of the instrument. 



FIG. 212. Ktihne's artificial eye. 



1. The Action of the Cornea. If the illuminated arrow be placed 

 approximately in the optic axis of the artificial eye, the rays of light 

 will be seen passing through the box and converging somewhat in their 

 progress. If the frosted glass screen be placed in the box, however far 

 back it be arranged, no image of the arrow will be obtained. If, how- 

 ever, a screen be placed some distance behind the box an image will be 

 formed. We have here illustrated the fact that without some specially 

 strong refracting medium in the eye, external objects would be focussed 

 behind the position of the retina and therefore not clearly visible. This 

 is the case after the operation for cataract in which the crystalline lens 

 is removed. 



2. The Action of the Crystalline Lens. Let the double convex lens 

 supplied be now placed in the box at the front end. This at once 



