396 PHYSIOLOGICAL PHYSICS. [Chap. xxix. 



direction of the dotted line AO. But let the plates 

 be rotated, the rays strike the plates obliquely and are 

 refracted. Thus, to consider only one plate, the ray 

 AO assumes the direction AO', and is displaced to the 

 right by the plate n. Similarly the plate n' will dis- 

 place the rays, and thus another image would be seen on 

 the left side of o, and at the same distance from it as o'. 

 A double image would be produced. The main part 

 of the figure shows an object AB viewed through the 

 ophthalmometer. By rotating the plates MN QP, AB is 

 seen as if double, and if the two images just touch one 

 another then the distance between the outer edges of 

 the double images is equal to twice the size of AB. 

 The size of the double image can be calculated from 

 the angle through which the plates have been turned 

 to make the images stand edge to edge. Connected 

 with the plates there is a circle on which is measured 

 in degrees the inclination of the plates. 

 The formula is 



sn a\ 



a = 2<? sin a (1 -- -. ) ; 



sin 2 a 



where d is the distance between the outer edges of 

 the double image, e is the thickness of the plates, n 

 their index of refraction, and o the angle through 

 which the plates were turned. 



A simpler method of using the ophthalmometer is 

 to place it at a given distance from a scale on which 

 are marked fractions of a millimetre. Turn the plates 

 and note the angle corresponding to certain distances 

 on the scale. In this way a table may be constructed 

 for the ophthalmometer, giving the size of the object 

 for a definite movement of the plates with the instru- 

 ment at the fixed distance from the object. 



To determine the size of the images reflected from 

 the cornea, a person is seated in a darkened room, at 



