346 



SCIENTIFIC AMUSEMENTS. 



the two lenses ; the distant object, AB, would form an inverted 

 real image, ab, on the other side of the convex lens were it not 

 that the rays tending to form this image are intercepted by the 

 concave lens, and so refracted as to produce upon the eye of the 

 observer the same effect as though they had been emitted by an 

 object, aft ; i.e., an erect virtual image, a/3, is formed. The linear 

 magnification or magnifying power is the ratio of the angle sub- 

 tended at the eye by the 

 virtual image aft to that 

 subtended by the actual 

 object, AB, and is approxi- 

 mately equal to the 

 quotient obtained by di- 



viding the focal length of 



Fig. 190. Galilean Telescope. 



the convex lens by that of the concave one ; e.g., if these lengths 

 be 8 and 2 inches respectively, the magnifying power will be about 

 4 diameters; if 12 and 1, about 12 diameters, and so on. 



Expt. 374. The Astronomical Telescope. This telescope prin- 

 cipally differs from the preceding form of Galileo's construction 

 in that the lens nearest the eye (eyepiece) is convex and of short 

 focal length, the two lenses being now placed at a distance apart 

 nearly equal to the sum of their focal lengths, instead of the 

 difference. As before, the magnifying power is nearly equal to the 

 quotient of the focal length of the glass furthest from the eye 

 (object glass), divided by that of the eyepiece ; so that for the 

 same magnification the astronomical telescope is longer than the 

 Galilean. 



Fig. 191 represents the formation of an image by this form of 

 telescope ; the object glass produces a real image, ab, situated 

 between the two lenses and inverted, the object glass acting here 



Fig. 191. Astronomical Telescope. 



precisely as the lens of a camera obscura (Expt. 370). The eye- 

 piece acts as a simple magnifying lens (Expt. 367), producing to the 

 eye of an observer at a virtual erect magnified image, aft, of 

 what serves to this lens as object, viz., the real image ab, formed 

 by the object glass ; as this is inverted with respect to the actual 



