40 On Measurement of Curvature and Refractive Index. 



rectness of this adjustment may obviously be determined at 

 the eyepiece. Let the glass plate form the bottom of a trough 

 in which the liquid may be placed, and let there be under this 

 a dish containing mercury. Now, there is a certain inclination 

 of the telescope at which the beam from the illuminated slit, 

 rendered parallel by the object-glass, is refracted at the free 

 surface of the liquid, and again at the surfaces of the glass 

 plate, so as to leave the latter vertically; then, striking the 

 mercury, it returns along the path whence it came, and may 

 be viewed by the eyepiece. Under these conditions the beam 

 of light on either side of the prism is at right angles to the 

 opposite side; therefore they make equal angles with the 



adjacent sides, and the prism is at minimum deviation. 



n 

 Calling the inclination of the telescope 6, we have sin# = //, sin^; 



therefore [i— \/ 2(1 + cos 6). The telescope may be first in- 

 clined on one side and then on the other, and half the angle 

 moved over taken as 6. I have found it utterly impossible to 

 get the sodium-line to keep still for a moment, or to be even 

 fairly defined, as every movement in the neighbourhood pro- 

 duces a tremor on the surface of the liquids, which, in the 

 Science Schools at Kensington at any rate, is so continuous 

 as to make it impossible to observe with accuracy. I should 

 have said that the top of the prism must be covered in with a 

 glass plate, to prevent the evaporation and consequent super- 

 ficial cooling of the liquid forming the prism, which causes 

 strias in the liquid, spoiling definition even more than the con- 

 tinuous tremor. 



Helmholtz and others have shown that, during accommoda- 

 tion of the eye for near objects, the cornea does not change 

 in curvature, the front surface of the lens becomes more 

 curved and advances, and the back surface does not appreciablv 

 change. The proof given is that the images of a light pro"- 

 duced by reflection from Ihe cornea and from the back 

 surface of the lens do not change, while that produced by 

 the front surface of the lens advances and becomes smaller. 

 Now it would appear at first sight that these observations 

 prove a flattening of the back surface of the lens during 

 accommodation ; for if it did not change in curvature, the 

 rays of light passing twice through the more curved front 

 surface would sooner come to a focus ; but since they do 

 not apparently sooner come to a focus, it would seem that a 

 flattening of the back surface must have occurred to counteract 

 the shortening influence of the more curved front surface. In 

 the case of ordinary lenses this would be so; but it so happens 

 that in the crystalline lens the focus by reflection is formed 



