OF OPTICS. 269 



But as this telescope inverts the image with respect 

 to the object, it gives an unpleasant view of terrestrial 

 objooto , and i s on i v fit for viewing the heavenly bodies, 

 in which we regcua ,.,. u-_:_ ^ nc .-, t ; nn Hpran<?e their 

 being inverted does not appear, on account of their 

 being round. But whatever way the object seems to 

 move, this telescope must be moved the contrary way, 

 in order to keep sight of it ; for, since the object is 

 inverted, its motion will be so too. 



The magnifying power of this telescope is as the 

 focal distance of the object-glass to the focal distance of 

 the eye-glass. Therefore, if the former be divided by 

 the latter, the quotient will express the magnifying power. 



When we speak of the magnifying of a telescope or 

 microscope, it is only meant with regard to the diameter, 

 not to the area or solidity of the object. But as the 

 instrument magnifies the vertical diameter, as much as 

 it does the horizontal, it is easy to find how much the 

 whole visible area or surface is magnified : for, if the 

 diameters be multiplied into one another, the product 

 will express the magnitude of the whole visible area. 

 Thus, suppose the focal distance of the object-glass be 

 ten times as great as the focal distance of the eye-glass, 

 then, the object will be magnified ten times, both in 

 length and breadth : and 10 multiplied by 10, produces 

 100 ; which shews that the area of the object will ap- 

 pear 100 times as big when seen through such a 

 telescope, as it does to the bare eye. 



Hence it appears, that if the focal distance of the eye- 

 glass, were equal to the focal distance of the object- 

 glass, the magnifying power of the telescope would be 

 nothing. 



This telescope may be made to magnify in any given 

 degree, provided it be of a sufficient length. For, the 

 greater the focal distance of the object-glass, the less 

 may be the focal distance of the eye-glass ; though not 

 directly in proportion. Thus, an object-glass of 10 



s 2 



