DOUBLE REFRACTION. 77 



By this duplication the weak double refraction of rock crystal 

 is rendered very sensible. 



(97) A prism of this kind is employed in the double 

 image micrometer, an ingenious and valuable instrument in- 

 vented by Rochon. It consists of a telescope, in which a 

 prism, such as we have described, is introduced between the 

 object-glass and its principal focus ; and thus two images are 



formed in the principal focus, whose interval is greater or 

 less, according to the distance of the prism from that point. 

 When the instrument is used, the prism is moved until the 

 two images appear in contact, and its distance from the focus 

 is then read on a graduated scale. The two angles in this 

 case having the same subtense, the visual angle of the object 

 is to the deviation produced by the prism, as the distance of 

 the prism from the focus is to the focal length. Now the di- 

 vergence of the two rays is constant for a given prism, and 

 may be determined either by calculation or experiment ; 

 consequently, the visual angle is deduced from the preceding 

 proportion. By this instrument Arago has determined the 

 apparent diameters of the planets with great precision. 



The same instrument has been also employed in war, to 

 determine the distance of an inapproachable object. Thus, if 

 it be required to ascertain the distance of the walls of a be- 

 sieged town, in order to know whether they are within the 

 range of shot, it is only necessary to measure by this instru- 

 ment the angle subtended by any object there whose 

 height is known approximately. The height of the object, 

 divided by the tangent of the angle, is the distance required. 



