8 S 



THE FUNCTION OF THE RETINA IN VISION. 





and uvea. If a diaphragm be placed in front of the eye, so that only the pupil 

 is free, the eyeground appears dark. In many animals the eyes have a bright- 

 green luster. These possess a special layer, the tapetum, or the membrana versi- 

 color of Fielding, which in the carnivora is composed of cells, in the herbivora of 



fibers; it lies between the chorio- 

 capillary layer and the stroma of the 

 uvea, yielding interference-colors, and 

 reflecting a considerable amount of 

 light, so that the eyes have a colored 

 luster. 



For examination of the anterior 

 chamber oblique illumination is em- 

 ployed. A bright beam of light, con- 

 densed by a convex lens, is thrown 

 obliquely into the eye, upon the point 

 to be examined, which appears clear 

 and distinct. The point thus illumin- 

 ated, for example a part of the iris, 

 can then be magnified and examined 

 with the help of a lens, or even of a 

 microscope. 



Czermak constructed the ortho- 

 scope (Fig. 297), by means of which 

 the eye is placed under water. A 

 small glass trough, one side of which 

 is removed, is filled with water and 

 pressed against the face, so that the 

 eye and the face form the sixth side of 

 the trough, and the cornea is covered 

 with water. As the index of refraction 

 of the water is the same as that of the 

 media of the eye, the rays pass out of 

 the eye unrefracted. Hence, objects 

 in the anterior chamber can be seen 

 directly, and appear as though outside 

 of the eye. A further advantage lies 

 in the fact that the objects are brought 

 closer to the observer's eye. The rays 

 from the point a of the eyeground would leave the eye parallel as b c, b c, 

 if the eye were surrounded by air. Seen under water, however, these rays a b, a b 

 continue in the same direction as far as d, d, where they are deflected from the 

 perpendicular on emerging from the water, that is toward d e, d e. The ob- 

 server's eye, looking in the direction e d, sees the point a closer, that is in the 

 direction e d a f ', consequently situated at of '. 



FIG. 297. Mechanism of the Orthoscope. 



THE FUNCTION OF THE RETINA IN VISION. 



The rods and cones are the only parts of the retina sensitive to 

 light; they alone are stimulated by the vibrations of the luminiferous 

 ether. This is confirmed by Mariotte's experiment, which shows that 

 the optic-nerve entrance, where rods and cones are absent, has no light- 

 perception. This is, therefore, called the blind spot. 



If the letter f (Fig. 279, p. 829) of the two black letters B and f is fixated with 

 one eye (the other being closed), so that its image falls on the fovea centralis 

 (n), and the image of B falls on the optic-nerve entrance (N), the letter B dis- 

 appears immediately. If three points, A f B, are drawn, and the eye fixes the 

 middle point f, B disappears, but the points A and f are visible. 



The optic-nerve entrance lies about 3.5 mm. to the inner side of the visual 

 axis in the retina. It has a diameter of i .8 mm. In the field of vision the horizon- 

 tal diameter of the blind spot measures apparently 6 56'; it lies from 12 8 1' 

 to 1 8 55 external to the point of fixation. This diameter would cover n full 

 moons, placed side by side, and would conceal a human face at a distance of more 

 than 2 meters. 



