ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



711 



forth in the outer tube. On the axis of the pinion is a milled head, 

 for turning the pinion and discs in duplicate, graduated on outer edge 

 and face with a scale in millimetres of radii and their equivalent in 

 dioptres. The effect of moving the prisms longitudinally is to approxi- 

 mate or separate the images of the mires as seen on the cornea. 



Fig. 141. 



In using the ophthalmometer, the operator, after the necessary adjust- 

 ments for height, obtains a clear image in the patient's eye of the mires 

 by the focussing adjustment. He then turns the tube 'horizontally 

 slightly to right or left until two images of the mires are seen in close 

 proximity (fig. 143). An outer image may be seen on either side of the 

 field of view, but these are always widely separated from the inner ones, 



