266 FERGUSON'S LECTURES. 



LECT. are no ^ coloured by reflection from a concave mirror, if 

 yy- it be ground to a true figure, as they are by passing 

 through a convex-glass, let it be ground ever so true. 



The adjusting screw on the outside of the great tube 

 fits this telescope to all sorts of eyes, by bringing the 

 small mirror either nearer to the eye, or removing it 

 farther : by which means, the rays 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 the longer of 

 meeting in points after reflection ; so that the first image 

 / K will be formed at a greater distance from the 

 large mirror, when the object is near the telescope, 

 than when it is very remote. But as this image must 

 be formed farther from the small mirror than its princi- 

 pal focus, n, this mirror must be always set at a greater 

 distance from the large one, in viewing near objects, 

 than in viewing remote ones. And this is done by turn- 

 ing 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 it 

 which is formed next the eye in the telescope. For, if 

 a man holds his finger or a stick between his bare eye 

 and an object, it will hide part (if not the whole) of the 

 object from his view. But if he ties 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 te- 

 lescope before, unless it covers the whole mouth of the 

 tube : for all the effect will be, to make the object ap- 

 pear dimmer, because it intercepts part of the rays. 

 Whereas, if he puts only a piece of wire across the in- 

 side of the tube, between the eye-glass and hits eye, it 

 will hide part of the object which he thinks he sees : 



