188 



touches its edge: then the side of the Uttle minor, cor- 

 responding witli that of the illuminated circle, where it is 

 in contact with the crescent, makes too great an angle, 

 with the axis of the instrument; and it must be reduced 

 to a right angle, by screwing forward the adjusting screw 

 of the little mirror, in that part, having previously with- 

 drawn the opposite screws. When the mirrors are thus 

 found to be rightly placed, and the eye-tube and lenses 

 are restored to their places; if the whole image, of the 

 great mirror, be not visible in the eye-hole, when this is in 

 the common axis of the instrument, then the lenses are 

 xlefective: either, some of them, or some of their sur- 

 faces, Qi- the tube they are fixed in, being inclined to the 

 iCommon axis; and, by this means, distorting the cone of 

 rays, from it Which irregularities must be rectified, be- 

 fore a true estimate £aa he formed, of the correctness of 

 the mirror's curvature. 



The distance from the smaller lens, at which is the 

 point of decussation of all the pencils of light, and the 

 place whcje the eye-hole ought to be, may also be most 

 easiiy and most accurately found, by directing the telescope 

 -to the sun, having taken oft" that part of the eye-tube be- 

 hind the lenses; and letting the light fall on a plane, 

 moveable back and forward, behind the second eye-glass, 

 and kept at right angles, to the axis of the specula and 

 lenses: the place, 'at which the image of the great mir- 

 ror, with the shadow of the little mirror described on it, 

 is seen most distinctly formed on the plane, ought to be 

 the place of tlife eye-hole. 



ON 



