Chap. XXIX.] 



THE OPHTHALMOMETER, 



395 



the one compensating for the other. The iris also aids 

 in diminishing the aberration. 



The ophthalmometer. It may be well before 

 concluding this chapter to describe briefly the principle 

 on which this instrument is constructed. It was 

 devised by Helmholtz for the purpose of measuring 

 the size of the images reflected from the surface of the 

 cornea or lens. Knowing the size of the images and 

 the distance of the object from the reflecting surface, 

 the radius of curvature of the surface can be calcu- 

 lated. 



The ophthalmometer consists of a tube in which are 

 placed one above another two similar plates of glass 

 with parallel faces. The glass plates revolve on a 

 vertical axis common to both, but, on turning the 



7 O 



screw, the plates revolve in opposite directions. Now 

 we have seen that rays of light falling perpendicularly 

 on a plate with parallel faces will 

 pass straight through without 

 deviation. If the rays fall ob- 

 liquely they will undergo deviation, 

 but will issue from the plate in a 

 direction parallel to that in which 

 they fell upon the glass. One end of 

 the tube T is directed towards the 

 object to be observed, and in the 

 other end is an eye-piece formed of 

 two achromatic lenses, through 



* o 



which the observer looks. The 

 principle of the instrument is illus- 

 trated in Fig. 177. In the figure 

 to the left hand, A represents 

 an object from which rays are 

 reflected to the ophthalmometer. 

 Suppose the plates not to have been revolved and 

 that the reflected rays fall perpendicularly upon 

 the plates, they will pass straight through in the 



Fig. 177. The Oph- 

 thalmometer. 



