THE SENSE OF SIGHT 721 



explanation of the process, especially when it was shown to undergo 

 changes when exposed to the action of light. But as the pigment is wanting 

 in the cones, and especially in the fovea, it cannot be considered essential 

 to distinct vision, although that it plays some important r61e in the visual 

 process is highly probable. It was observed by Van Genderen Stort, 

 that when an animal is kept in darkness some time before death, the cones 

 are long and filiform; but if the animal has been exposed to light, they are 

 short and swollen. It was discovered by Boll that if an animal is kept in 

 darkness an hour or two before death the pigment is massed at the ends of 

 the rods .and cones, but after exposure to light it becomes displaced and 

 extends over and between the rods almost to the external limiting mem- 

 brane. These conditions are represented in Fig. 309. 



The Eye a Living Camera. In its construction, in the arrangement 

 of its various parts, and in their mode of action the eye may be compared 

 to a camera obscura. Though the comparison may not be absolutely exact, 

 yet in a general way it is true that there are many striking points of simi- 

 larity between them; e.g., the sclera and chorioid may be compared to the 

 walls of the camera ; the combined refracting media to the component glasses 

 of the lens, the action of which results in the focusing of the light rays; the 

 retina to the sensitive plate receiving the image formed 

 at the focal point; the iris to the diaphragm for the 

 regulation of the amount of light to be admitted, and 

 for the partial exclusion of those marginal rays which 

 give rise to spheric aberration; the ciliary muscle to the 

 adjusting screw, by means of which the image is brought 

 to a focus on the sensitive plate, notwithstanding the 

 varying distances of the object from the lens. The FIG. 310. RETINA OP 

 presence of the visual purple in the rods of the retina ^ ^^WIND^FWR 

 capable of being altered by light makes the compari- METERS DISTANT a. 

 son still more striking. Yellow spot, b, b. 



Kuhne even succeeded in obtaining a fixed image SSkb^-^Sfif 

 or an optogram of an external object in a manner 

 similar to that by which an image is fixed on the sensitive plate of a 

 camera. An animal is kept in the dark for about ten minutes in 

 order to permit the retinal pigment to be completely regenerated. 

 The animal, with the eyes covered, is then brought into a room with 

 a single window. While the head is steadily directed to the window, 

 the eye is exposed for several minutes. The eyes are again covered, the 

 animal killed, and the eyes removed by the light of a sodium flame. The 

 retina is then placed in a 4 per cent, solution of alum. In a short time the 

 image of the window, the optogram, will be fixed (Fig. 310). That portion 

 of the image corresponding to the window lights will be quite bleached in 

 appearance from the action of the light on the pigment, while that corre- 

 sponding to window bars will have the usual color of the retina. Dur- 

 ing life the regeneration of the visual purple must take place with extreme 

 rapidity if a similar change takes place with the formation of each image. 

 The visual purple is believed to be derived from a pigment secreted by the 

 layer of pigment cells. 



Binocular Vision. Though two images are formed, one on each 

 retina, when the eyes are directed to a given object, there results but one 

 4 6 



