842 THE SENSE OF SIGHT 



ophthalmoscope is of no avail, because the perfectly clear retina 

 lies here upon the dark background of the choroid. Various other 

 means, however, are at our disposal to show that this pigment is a 

 chemical entity serving a particular purpose. Thus, it will be found 

 that the retina of an eye which has been protected against light for 

 some time, possesses a purple color, while one which has been exposed 

 to strong daylight, is entirely colorless. The purple color of the 

 former soon becomes yellowish and then disappears completely. 

 This bleaching property of the visual purple enables us to employ the 

 retina in the manner of a photographic plate, but naturally, the objects 

 to be taken must show sharp contrasts. Most commonly, we employ 

 the eye of a rabbit or frog which has been directed for a brief period of 

 time toward a window, preferably one with many cross-bars. 14; is 

 then bisected and immersed in a 4 per cent, solution of alum which 

 temporarily fixes this inverted image of the window. A retinal 

 photograph of this kind is known as an optogram (Fig. 444). 



i 



FIG. 444. OPTOGRAM IN EYE OF RABBIT. 



1. The normal appearance of the retina in the rabbit's eye: a, The entrance of the 

 optic nerve; b, b, a colorless strip of medullated nerve fibers; c, a strip of deeper color 

 separating the lighter upper from the more heavily pigmented lower portion. 2 shows 

 the optogram of a window. (Howell.) 



It has previously been mentioned that the visual purple is pro- 

 truded from the pigmental epithelium in the form of delicate processes 

 which invade the layer of the rods and cones and closely invest the 

 outer limbs of the former. It cannot surprise us, therefore, to find 

 that the retina of the light adapted eye is closely adherent to the choroid, 

 while that of the dark adapted eye may be easily peeled off. Further- 

 more, a retina which has been bleached, does not regain its original 

 color unless it is allowed to remain in contact with the pigmented 

 epithelium. These data clearly prove that the choroidal pigment 

 serves as the mother-substance of the visual purple, its function 

 being to supply this sensitive substance to the outer limbs of the rods 

 as quickly as it is reduced by the light rays. 



The visual purple may be extracted from the retina by means of 

 solutions of bile salts. It will be remembered that the latter possess 

 the power of quickly abstracting the hemoglobin from the red blood 

 corpuscles. These actions are very similar, in the present case the 

 visual pigment being liberated from its combination in the rods. The 



