REFLECTING TELE5C0PE, 55 



U necessary, without any injury to the surface of the po- 

 lisher, if it be previously wetted, to prevent the splinters of 

 the pitch from sticking to it ; which may be washed off, by a 

 soft brush or pencil, from the polisher, it being immersed in 

 water. 



Since, in the Gregorian telescope, the defect of ^g^re or j^j^ ^g^^p^. ^j^^^. 

 curvature, from that of a conoid, in one of the mirrors, iray the small spe- 

 be compensated by a contrary curvature in the other j and ^"^""? should 

 since, in either of the mirrors, whose breadth is given, the 

 degree of variation in its figure, from that of a sphere, ought 

 to be so much the greater, as its focus, or radius of curvature, 

 is shorter; it will, on this account, be far more difficult, to 

 effect a proper figure of the small mirror in this telescope, 

 than of the large one; because the former must be a greater 

 segment of the sphere, than the latter. For which reason, 

 instead of making the one of an eUiptic, and the other of a 

 parabolic form, I imagine it would (with the exceptions 

 before mentioned) be more proper to rest content with a sphe- 

 rical form in the little mirror, (by which means, several of 

 these latter, being fastened, with cement, beside each other, 

 on the same handle, might be accurately and easily ground 

 and polished together, on one tool and polisher, made suffi- 

 ciently large); and to employ the great efforts on the large 

 mirror, in rendering it of an hyperbolic form ; which is not at 

 all more difficult than it is to make it parabolic : for, on ac- 

 count of the small extent of surface of the little mirror, it is 

 very difficult to govern and regulate its motion and pressure, 

 so as to communicate to it any certain figure, if polished by 

 itself singly ; as it must be, when it is to be of any other than 

 a spherical form. Yet, even this rnay, by an intelligent and 



dexterous artist, be accomplished, to a considerable degree of !'"'•, '^^ ^®' 

 r . . , , , T , noidal figure 



periectjon, m the manner above mentionned, as 1 have re- maybe given, 



Q^eatedly experienced ; though the process is much more easy 



and certain, in figuring the large mirror (under that limitation 



ofits size before intimated): for the greater the surface to be 



pohshed is, the less will any inequality of pressure, in the 



operation, alter the form of the mirror, or the polisher ; such 



inequality, being a part only of the motive force employed ; 



and the more extensive the surface is, the less proportion does 



the motive force bear to the force of cohesion, which tends to 



preserve an uniformity of pressure in the mirror;, ai>d of ligurQ 



