672 DEMONSTRATING REFRACTIVE EYE DEFECTS [CH. XV 



diopter lens gives a smaller image than the 4 diopter lens* (see also 

 fig. 389 B. C.). 



B 



FIG. 402. APPEARANCE OF THE SCREEN IMAGE WITH UNLIKE REFRACTION 

 IN THE Two EYES AND WITH LIKE REFRACTION. 



A Image of a 3 diopter left eye and a 4 diopter right eye; the 4 diopter 

 eye is in focus. 



B Image when a i diopter convex spectacle is added to the 3 diopter left 

 eye, making it like the right eye. 



932a. In the experiment showing anisometropia each half of the double 

 lens projects both images, but when the light is properly centered and in the 

 correct position to give the large illumination on the lens (fig. 400), each half 

 lens projects a much more brilliant image of its own side, hence the fainter 

 image of the opposite side is overwhelmed and overlaid so that only one image 

 shows on each side. If the light is not in a good position, both images show 

 and that spoils the effect. 



This demonstration with two half lenses was fully successful only when a 

 right-angle, direct current arc lamp was used as a source of light. 



By using a I diopter concave lens to reduce the 4 diopter half lens to a 3 

 diopter power, the right eye image can be made sharp when the lantern slide 

 is in position to make the left eye image sharp, and the right eye image blurred. 

 It is a little more satisfactory to work with the 4 diopter lens, however, and to 

 add the I diopter convex lens to the 3 diopter left lens. 



