348 



HOW TO OBSERVE THE HEAVENS. 



these diverging rays which will enter the eye is limited by the magnitude of 

 the pupil. But before they reach the eye they may be received upon a glass 

 lens of a convex form (fig. 10), which will have the effect of collecting them 

 into a space less in magnitude than the pupil of the eye. If the eye be placed 

 where the rays are thus collected, all the light transmitted by the lens will 

 enter the pupil. 



Now let us see what will thus be effected. The object-lenses of some 

 telescopes are above ten inches diameter. Most common telescopes, however, 

 are much smaller. Suppose, for example, the lens has five inches diameter, 

 and taking the diameter of the pupil at a quarter of an inch, the ratio of these 

 diameters would be 20 to 1, and consequently the superfices of the lens would 

 be four hundred times greater than the opening of the pupil, and would there- 

 fore admit four hundred times more light. A lens of ten inches diameter, 

 having a surface four times greater than one of five inches, would therefore 

 admit sixteen hundred times more light than the pupil. 



What is effected by a convex lens, as represented in fig. 10, may also be 

 accomplished by a concave reflector, as represented in fig. 11. In the one 

 case the light is transmitted through the surface which receives it in the 

 other it is reflected from it. In the one case the eye which receives the light 

 is placed behind the lens and directed toward the object in the other it is 

 placed before the reflector, and looking in a direction opposite to that of the 

 object. The observer turns his face to the object in the one case, his back in 

 the other. 



But in the practical realization of this, there are two circumstances to be 

 taken into account. First There is no body which is capable of perfectly 

 transmitting or reflecting light ; that is to say, there is none which will either 

 transmit or reflect all the light which strikes upon it. Light is then lost in a 

 greater or less proportion whenever refraction or reflection takes place. If 

 this loss of light were in the same proportion as that of the magnitude of the 

 lens or reflector to that of the pupil, then nothing would be gained by the op- 

 tical expedient above explained. But such is not the case. Although a cer- 

 tain proportion of the rays incident on a lens fail to pass through it, and a much 

 greater proportion of those incident on a reflector fail to be regularly reflected 

 from it, yet even the highest of these proportions of loss is incomparably less 

 than the proportion in which the light is condensed. 



Secondly The eye in general has been so constituted by its Maker, as to 

 be capable of producing distinct vision only when the rays which enter the 

 pupil from any point of a visible object, are parallel, or nearly so. Now, when 

 the rays are collected by either of the expedients above explained, they will 

 first converge to a focus, and afterward diverge from it. If the eye be placed 

 within the focus, it will receive converging rays, if without it, diverging rays ; 

 and in either case vision would be indistinct. This difficulty is surmounted 

 by placing between the eye and the rays collected into a focus a small lens, 

 which may be either convex or concave, and which is so adapted that it will 

 render the rays parallel, without affecting them in any other way. 



Such is the combination which forms an astronomical telescope. 



By an instrument of this kind, then, we accomplish what is equivalent to an 

 enlargement of the pupil, and objects which transmit light so attenuated as to 

 be incapable of affecting the retina with sufficient energy to produce vision, 

 may by such means be rendered visible. If, for example, the quantity of light 

 received by the pupil from any distant star be ten times less than that which 

 would be necessary to produce vision, such a star will become visible in a 

 telescope whose object-glass is capable of condensing the light so as to render ' 

 it ten times more intense. 



