OPTICS. 507 



are made to diverge a little for short-sighted eyes, 

 or to converge for those of a long sight. 



The nearer an object is to the telescope, the 

 more its pencils of rays will diverge before they 

 fall upon the great mirror, and, therefore, they will 

 be longer off meeting in points after reflection ; so 

 that the first image I K, will be formed at a greater 

 distance from the large mirror, when the object is 

 near the telescope, that when very remote. But 

 as this image must be formed farther from its small 

 mirror than its principal focus n, this mirror must 

 be always set at a greater distance from the large 

 one in viewing near objects, than in viewing re- 

 mote ones : and this is done by turning the screw 

 on the outside of the tube, until the small mirror 

 be so adjusted, that the object (or rather its image) 

 appears perfect. 



In looking through any telescope towards an 

 object, we never see the object itself, but only that 

 image of which is formed next the eye in the tele- 

 scope ; for, if a man hold his finger or a stick be- 

 tween his eye and an object, it will hide part, if 

 not the whole of the object from his view. But if 

 he tie a stick across the mouth of a telescope, 

 before the object-glass, it will hide no part of the 

 imaginary object he saw through the telescope 

 before, unless it cover the whole mouth of the 

 tube : for all the effect will be to make the object 

 appear dimmer, because it intercepts part of the 

 rays. Whereas, if he put only a piece of wire 

 across the inside of the tube, between the eye-glass 

 and his eye, it will hide part of the object which 

 he thinks he sees : which proves that he sees not 

 the real object, but its image. This is also con- 

 firmed by means of the small mirror L, in the re- 

 flecting telescope, which is made of opaque metal, 



x 2 



