Vision and Optical Glasses. 197 



mirror, to R ; there they are met by the plano- 

 convex lens hi, which brings them to a conver- 

 gence at S, and paints the image in the small tube 

 of the telescope close to the eye. Having by this 

 lens, and the two mirrors, brought the image of 

 the object so near, it only remains to magnify this 

 image by the eye-glass Jcr ; by which it will ap- 

 pear as large as zy. 



To produce this effect, it is necessary that the 

 large mirror should be ground so as to have its 

 focus a little short of the small mirror, as at q ; 

 and that the small mirror should be of such con- 

 cavity as to send the rays a little converging 

 through the hole o ; that the lens hi should be of 

 such convexity as to bring those converging rays 

 to an image at S ; and that the eye-glass Icr should 

 be of such a focal length, and so placed in the 

 tube, that its focus may just enter the eye through 

 the small hole in the end of the tube. 



To adapt the instrument to near or remote ob- 

 jects, or rather to rays, that issue from objects 

 converging, diverging, or parallel, a screw, at the 

 end of a long wire, turns on the outside of the 

 tube, to take the small mirror nearer to, or fur- 

 ther from, the large mirror ; and so as to adjust 

 their foci according to the nearness or remoteness 

 of the objects. The sun-glass at the end of the 

 small tube should be unscrewed, when any other 

 object, except the sun, is looked at. This pecu- 

 liar construction of the reflecting telescope is 

 called the Gregorian telescope, from the name of 

 its inventor. 



