C hap. 8 .] Principle of the telefcope. 2 5 1 



as q F/> to q Ep, that is, as q E to q F, or as the focal 

 length of the eye glafs to the focal length of the object- 

 glafs *. 



By this fimple combination of glafTes an obj eel: ap- 

 pears inverted 3 but this is of no confequence to altro- 



* < When the diftance of the object is very confiderable, the 

 effects may all be referred to the fame diftance, and a telefcope 

 may be faid to enlarge an object jaft as many times as the angle 

 under which it reprefents it is greater than th:it under which it ap- 

 pears to the r: j .ked eye. Thus the moon appears to the naked eye 

 under an angle of about half a degree; confequently a telefcope 

 magnifies 100 times, if- it reprefents th? moon under an angle of 

 50 degrees ; if it magnified 200 times, it would exhibit the moon 

 under an angle of 100 degrees ; and the moon would appear to 

 occupy more than half the vifible heavens, of which the whole ex- 

 tent is or.ly 180 degrees. 



' It is a common exprelTion, that telefcopes bring objects nearer; 

 but this exprefiion is equivocal, admitting of two different fignifi- 

 cations. The one is, that, looking through a telefcope, we efti- 

 mate an object to be as much nearer to us as it is magnified by 

 the telefcope. But I have already fhewn, that we can form no 

 certain eftimate of the diirance of an object but by the judgment, 

 and that our judgment deceives us when the objects are beyond a. 

 certain distance ; and in the prefent inftance, lofmg all thofe fub- 

 jects of comparifon on which it is founded, will deceive us more. 

 The other meaning applied to, the expreffion is, that the telefcope 

 reprefents the objects as large as they would appear if we were fo 

 much nearer to them : this latter meaning is more conformable to 

 the truth than the preceding, for the nearer we approach to an 

 objefl the larger is the vifual angle. When you look, however, 

 at a well-known object, as a man, at a great diftance, and he is 

 feen under a larger angle, we are led to think him fo much nearer, 

 when he would really appear under a greater angle; but with 

 refpect to objects lefs known, as the fun and moon, there can be 

 no eilimation of diftance. 



' One principal end of telefcopes is to enlarge or multiply the 

 3Pgle under which objects appear to the naked eye, and they 

 are eftimateil according to this effect, and are faid to magnify 

 live, ten, or any other number of times, according to the nature 

 and cqnftruction of the telefcope.' Adams's Le<3. vol. ii. p. 483. 



nomers, 



