too 



RETLrCTING TELESCOPE. 



«dge; then the side of the little mirror, corresponding with 

 that of the illuminated circle, where it is in contact with 

 the crescent, makes too great an anp:le, with the axis of the 

 instrnment; and it must be rediced to a right nnjile, by- 

 screwing forward the adjusting screw of the liftle mirror, 

 in that part, having previously withdrawn the opposite 

 -r- and the eye screws. When the mirrors are thus found to be ri2;htly 

 lenses after- placed, and ^lie eye-tube arid lenses are restored to their 

 waj-dsset. places; if the whole image, of the great mirror, be not visi* 



ble in the eye-hole, when this is in the common axis of the 

 instrument, then the lenses are defective ; either, some of 

 them, Qr some of their surfaces, or the tube they are fixed 

 in, being inclined to the common axis ; and, by this means, 

 distorting the cone of rays, from it. Which irreg ilavities 

 must be rectified, before a true esHmate can be formed, of 

 the correctness of the mirror's curvature, 

 t'osition of the The distance from the smaller lens, at which is the point 

 <ye- o e. ^^ decussation ot all the pencils of light, and the place 



where the eye-hole ought to be, may also be most easily and 

 most accurately found, by directing the tele^G(jpe to the 

 sun, having taken off that part of the eye-tube behind the 

 lenses; and letting the light fall on a plane, movcabh- back 

 and forward, behind the second eye-glass, and kept at i ight 

 angles, to the axis of the specula and lenses ; the place, at 

 which the image of the great mirror, with the shadow of 

 the little mirror described on it, is seen most distinctly 

 formed on the plane, ought to be the place of the eye- 

 hole. 



