670 TEXT-BOOK OF PHYSIOLOGY. 



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. 330). 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 corres- 

 ponding 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 

 sensation. If the direction of either visual axis be changed by pressure on 

 the eyeball, there arise two sensations, and the object appears to be doubled. 

 The reason assigned for this, in the first instance, is that the two images 

 fall into the foveae, on two corresponding points; while in the second instance 

 they fall on non-corresponding points. It would appear, therefore, that for 

 the purpose of seeing an object singly when the eyes are directed toward 

 it, the rays emanating from it must fall on corresponding parts of the retina. 



As all portions of the retina are sensitive 

 to light, though in varying degrees, it is 

 not essential that the images always fall 

 in the foveae. The parts of the retinae 

 which correspond physiologically are 

 shown in Fig. 331. In this figure the 

 AREAS OF THE retinal area is divided into quadrants 

 R ETINA . by vertical and horizontal lines of sepa- 



ration, as they are termed. If one retina 



is placed in front of or over the other, it will be found that the quadrants 

 bearing similar letters cover each other. So long as the rays of light, 

 entering the eye, fall on corresponding areas the sensation of but one 

 object arises. If, however, they fall on non-corresponding areas, two sen- 

 sations arise. Normal binocular vision enlarges very considerably the area 

 of the visual field, permits of a better estimation of the size and distance 

 of objects, enables the mind to form more readily a perception of depth, 

 and increases the intensity of sensations. 



The Horopter. When the eyes are in a so-called secondary position 

 that is, in a position in which the visual axes are converged and directed 

 to a point in front of and in the middle plane of the body it will be found 

 on examination that rays of light from a number of other objects enter the 

 eye, pass through the nodal point, and fall on corresponding parts of the 

 two retinae and give rise to but single images. All such points lie, for the 

 horizontal line of separation, on a line termed the horopter. The form of 

 this line is that of a circle which passes through the fixation point and the 

 two nodal points. Any object on the horopter will give rise to but a single 

 image. This is shown in Fig. 332, in which the objects I, II, III project 

 their rays into both eyes and upon corresponding areas. 



In addition to the horopter for the horizontal line of separation, there 



