REFLECTING TELESCOPE. "^99 



across the inouth of the tube : then let the little mirror be 

 replaced, and the eye-tube taken out, and let the telescope 

 be directed to the sun's centre ; which may be done, by the 

 help of the little dioptrical telescope, called a finder^ affixed 

 to the instrument, if it be furnished with such ; or otherwise, 

 may be effected, with sufficient exactness for this purpose, 

 by pointing the telescope to the sun, so as that the shadow of ^jj, ^^ _^ 

 the little mirror may coincide with the hole, in the great - '1. > ,.i 

 mirror ; which will be, when the great tube is so placed, aft 

 to project its shadow of a circular form. In these circum- 

 stances, let the light, reflected from the little mirror, through 

 the round perforation in the great one, be received upon a 

 plane, placed at some distance behind the latter, at right 

 angles to the axis of the tube. The light will occupy a cir- 

 cular area on the plane ; and, if the centre of the light be 

 coincident with that of the shadow of the little mirror, this 

 mirror is not only parallel to the great one, but both are 

 duly adjusted, at right angles, to the axis of the tube ; 

 which, also, then corresponds with their axes. But, be- 

 cause the little mirror and its shadow, and also the cone of 

 light, reflected from this mirror, are of greater breadth than 

 the perforation in the great one; the boundaries of the re- 

 iiccted light, and those of the shadow, cannot be seen 

 wholly on the plane, through the hole in the great mirror, 

 in any one position of tl\e telescope. Let, therefore, the 

 axis of the telescope be a little diverted from the centre of 

 the sun, till the shadow of the edge of the little mirror 

 falls within the hole in the great one : by which, some di- 

 rect light will pass through, next the shadow, and appear on 

 the plane, in form of a crescent: and, at the same time, the 

 circle of the reflected light of the sun will have moved across 

 the shadow; till, by a certain degree of obliquity, in the 

 direction of the telescope, the edge of the circle of the re,, 

 fleeted light will be in contact, externally, with the crescent 

 of the direct light. And, if the crescent be ahvays of the 

 same breadth, Avhen this contact takes place, on every side, 

 by a diverting of the telescope, from the centre of the sun, 

 successively, in every direction ; then both the mirrors arc 

 parallel, and at right angles, to the axis of the instrument. 

 But, if the crescent be broader, in any certain position of 

 the tube, when the circle of reflected light just touches its 



J 2 iHW' 



