260 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in 



motions can be given, so that the object can be kept in the field of 



view for a long time. 



From what we have already said of the magnifying power of 



an astronomical refractor, it follows that it depends for the same 



instrument, or rather for the same object-glass, on the eye-piece. 



And indeed, the same object-glass gives various magnifying powers 



when different eye-pieces, with shorter or longer focus, are used. 



Theoretically speaking, the power of a telescope would be un- 



limited, did not other considerations, on which we will say a few 



words, come in. 



The quality and power of a telescope depend principally on the 



object-glass. In the first place it is indispensable that the material of 



which it is composed be as pure as possible, and that the glass be free 



from bubbles and striae. 



The grinding and polishing of the surface are also of great import- 



ance, and it is on their perfection that the sharpness of the images 



which are formed at the focus depends. 



Now, with equal perfection in the above qualities, the object-glass 



with the greatest diameter, and the longest focal length will allow 

 the greatest magnification. The brightness of the 

 virtual image depends in the first place on the 

 brightness of the real image, and consequently on 

 the quantity of luminous rays which contribute to 

 form it. This depends upon the size or aperture of 

 the object-glass. As the magnifying power of the 



FIG 199.-A portion of eye-piece spreads the light over a larger space, the 



the constellation Ge- J 



ike'i eye 11 with the v ^ r t ua l image is weakened and rendered indistinct in 

 proportion as this magnification is greater, unless 

 the rays- proceed from luminous points of imperceptible dimensions, 

 like the- stars. In this case the loss of light due to magnification is 

 slight, and the brilliancy obtained is in the ratio of the squares of 

 the aperture of the object-glass and the pupil of the eye. In this 

 manner with a lens of large aperture, the number of the stars seen 

 in a limited space of the sky is increased considerably, as repre- 

 sented in Figs. 199 and 200. The one shows a portion of the heavens 

 in the constellation Gemini, in which the stars seen with the naked 

 eye are seven in number ; by using a lens of 27 centimetres aperture 

 M. Chacormac has counted 3,205. If we allow 6 millimetres for the 



