SIGHT. 169 



What is the retinal red? 



When the retina of a recently-killed animal is examined it is 

 colorless, but during life or if extracted without exposure to light, 

 it is of a purple-red hue, and the color is found in the rods of the 

 retina. It is derived from the pigment of the deeper part of the 

 retina. It is the " retinal red " or " visual purple," as it is variously 

 named, which one sees in the reflex of the retina. 



What effect has exposure to light upon the retinal red? 



It destroys it, and for this reason it was long unknown. It dis- 

 appears after a brief exposure to sunlight, about half a minute. 



How may the retinal red be seen? 



By throwing a beam of light into the eye by a mirror, as by the 

 ophthalmoscope, a red glow is observed in the pupil. This is called 

 the retinal reflex. 



What are optograms? 



Pictures which appear upon the retina after exposure to light. 

 They are due to the fact that an exposure to light bleaches the 

 retinal red, leaving it dark in the shaded portions. 



How are optograms obtained? 



The eye is removed from an animal in a dark room and kept in 

 a covered box until exposed to a brightly illuminated skylight or 

 window for some minutes. The eye is then replaced in the dark 

 room and the retina examined. It will be found that the panes of 

 the window are shown in light color, while the sash is in dark out- 

 line. This soon fades on exposure to daylight, but if the retina is 

 dried in the dark the optogram is much more durable. 



Is the pigment of the retina essential to vision ? 



No, but it is of considerable use in rendering the eyeball a dark 

 box. which is of importance here as in all optical instruments. 



How is the real image formed by a double convex lens? 



It is an inverted image at the point of focus of the lens if the 

 luminous object is placed at a distance (Fig. 50). Referring to 

 this figure, it will be seen that the rays originating at A will be 

 twice refracted, once by the lens and again in leaving it, so that all 

 rays from A reaching the lens are joined at a. The same is true for 

 B and b. Therefore a screen placed at the focus, F, will receive an 

 inverted image, a b, of the luminous object, A B. 



