OF OPTICS. 263 



At the bottom of the great tube TTTT is placed LECT 



VII. 



the large concave mirror DUVF, whose principal 

 focus is at m ; and in its middle is a round hole P, op- 

 posite to which is placed the small mirror L, concave 

 towards the great one ; and so fixed to a strong wire M, 

 that it may be moved farther from the great mirror, or 

 nearer to it, by means of a long screw on the outside of 

 the tube, keeping its axis still in the same line Pmn 

 with that of the great one. Now since, in viewing a 

 very remote object, we can scarcely see a point of it 

 but what is at least as broad as the great mirror, we 

 may consider the rays of each pencil, which flow from 

 every point of the object, to be parallel to each other, 

 and to cover the whole reflecting surface DUVF. 

 But, to avoid confusion in the figure, we shall only draw 

 two rays of a pencil flowing from each extremity of the 

 object into the great tube, and trace their progress, 

 through all their reflections and refractions, to the eye^ 

 at the end of the small tube tt, which is joined to the 

 great one. 



Let us then suppose the object A B to be at such a 

 distance, that the rays C may flow from its lower ex- 

 tremity B, and the rays E from its upper extremity A. 

 Then the rays C, falling parallel upon the great mirror 

 at D, will be thence reflected, converging in the direc- 

 tion D G ; and, by crossing at /, in the principal focus 

 of the mirror, they will form the upper extremity / of 

 the inverted image IK, similar to the lower extremity 

 B, of the object A B : and passing on to the concave 

 mirror L (whose focus is at n) they will fall upon it at 



