OHAP. iv.] THE TELESCOPE. 255 



II. THE INVERTING TELESCOPE. 



We now come to an instrument of slightly different construc- 

 tion, the refracting telescope, generally used in the present day for 

 surveying and astronomical observations. This- instrument consists 

 essentially in a system of two converging lenses : the object-glass, 

 giving a real and reversed image of the object ; the other, the eye- 

 piece, magnifying the first, but preserving its inverted position. As 

 a matter of course the two lenses are both compound so as to produce 

 achromatic images. By the help of Fig. 193, the path of the luminous 

 rays in this instrument may be traced, and we shall easily see how 

 it differs from Galileo's telescope. 



The rays starting from the upper extremity of the object, supposed 

 to be situated at an infinite distance, form a parallel beam 1, 2, until 



FIG. 193. Path of the luminous rays in the inverting telescope. 



they reach the object-glass 0. On passing through this latter, where 

 they are refracted, they form, by their convergence at a, an image of 

 this extremity. In the same way the beam 3, 4, coming from the 

 lower part produces a real image b. Thus we have a reversed image 

 of the object at the principal focal distance of the object-glass, at ab. 

 This image the magnifying glass or eye-piece O', magnifies, at A'B', 

 that is, at a distance from the eye equal to the distance of distinct 

 vision. 



As in Galileo's telescope, the magnification depends upon the ratio 

 between the focal distances of the object-glass and of the eye-piece. 

 Therefore, the longer the focus of the object-glass, and the shorter the 

 focus of the eye-piece, the greater becomes the linear magnification. 



