256 PRACTICAL PHYSIOLOGY 



line of regard about a foot from the eyes, a double image of the finger 

 will be seen. In this case the images of the finger will fall upon non- 

 corresponding parts of the retina, and hence the images will not combine 

 to form a single sensation. 



EXPERIMENT I. Place a rod vertically about two feet from the eyes. 

 Adjust the vision for a clear image of the rod. Then hold up a finger 

 in the binocular line of regard about twelve inches from the eyes. A 

 double image of the fingers will be seen. Close the left eye, the right 

 image will disappear. Then accommodate for the finger, and a double 

 image of the rod will be seen. Close either eye, and the image on the 

 same side will disappear. 



This experiment may also be performed with the material in the 

 Milton Bradley Pseudoptic^, Section I., Experiment I. 



The double images seen, the above experiment may be crossed or 

 uncrossed. If crossed they are spoken of as heteronymous images, if 

 uncrossed, as homonymous images. 



In general, if the optic axes of the two eyes converge towards a 

 certain point, and a circle be described passing through this point and 

 the two centres of rotation of the eyes, then an object outside the circle 

 will produce homonymous images, and an object inside the circle, 

 heteronymous images. With a definite point of regnrd, then, it 

 will be found that if a circle be described through this point as 

 above, objects lying on this circle will be seen single. Such a 

 circle is called a horopteric circle, and the complete surface (inter- 

 sected as above by a horizontal plane, forming a circle) is referred to 

 as a horopter. 



Double images of single lines may be shown in performing the 

 Experiments II. and III, Section I., of the Milton Bradley Pseu- 

 doptics. 



When double images lie, not symmetrically with regard to the 

 line of regard, but both to one side of that line, that nearer the line of 

 regard is the more distinct, and the other is hardly discernible. 



[ADVANCED EXPERIMENT. Fix the eyes on some remote object, and 

 hold a pencil about six inches from the right eye and about two inches 

 to the right of a line passing from that eye to the remote object. The 

 image falling upon the right retina will alone be appreciated. Close 

 the right eye, and the second image will also be observed. 



In general the image falling upon the nasal side of one retina will 

 dominate over that falling on the temporal side of the other retina.] 



3. Binocular Fusion of Dissimilar Images. If two partially dis- 

 similar images, or at any rate not absolutely identical images, fall upon 



