CHAP, iv.] THE TELESCOPE. 251 



A refracting telescope, or, as it is termed shortly, a refractor, like 

 a compound microscope, is composed of two essential parts two 

 systems of lenses. The one nearer the object is called for this reason 

 the objective or object-glass : this is generally a biconvex lens with a 

 long focus, which produces a real and inverted image of the object. 

 The eye is applied to the other system of lenses called the eye-piece : 

 this is a simple or compound magnifying-glass, by which the image, 

 which is in a certain measure magnified, is examined. 



In the first telescopes the eye-piece was a biconcave lens, as we 

 have already seen ; the inverted image formed by the object-glass is 

 corrected in this system, as will be seen by the path of the luminous 

 rays represented in Fig. 189. The object-glass O gives at its focus, 

 which for very distant objects is the principal focus of the lens, a 



FIG. 189. Path of luminous rays in Galileo's telescope. 



real image la of the object observed. This image is inverted, which 

 may be proved by letting it fall on a screen. The bi-concave 0' being 

 placed between the image and the object-glass causes the luminous 

 rays to diverge and thus prevents the formation of the real image. 

 To the eye, into which these rays penetrate after leaving the eye- 

 piece, they appear to come from the points A' and B' situated on their 

 optic axes at their points of convergence. This gives a virtual erect 

 image AB', which is well defined if the lenses are arranged so that 

 this image is formed at the distance of distinct vision. There is an 

 essential difference between the magnifying power of refracting or 

 dioptric telescopes and that of microscopes. In these latter instru- 

 ments the magnified image is larger than the object itself, that is 

 to say, the angle subtended by the image is greater than the angle 



