us Dr.HerfcMsfelefcepe. [Book III. 



latter, and the quotient will exprefs the magnifying 

 power *. 



The immenfely powerful telefcopes of Dr. Herf- 

 chel are on a different conftruction. This afTiduous 

 aftronomer has made feveral fpeculums, which are fo 

 petfect as to bear a magnifying power of more than 

 fix thoufand times in diameter on a diftant object f . 

 The object is reflected by a mirror as in the Gregorian 

 telefcope, and the rays are intercepted by a lens at, 

 a proper diftance, fo that the obferver has his back 

 to the object, and looks through the lens at the mirror. 

 The magnifying power will in this cafe be the fame as 

 in the Newtonian telefcope, but there not being a fe- 

 cond reflect or, the brightnefs of the object viewed in 

 the Herfchelian is greater than that in the Newtonian 

 telefcope. 



There are feveral amufing optical deceptions, which 

 are effected by a proper combination of plane or con- 

 vex glaffes. My limits will not admit the notice of 

 more than two of the amufing kind, namely^ the ma- 

 gic lanthorn and the camera obfcura. The former 

 is a microfcope upon the fame principles as the folar 

 microfcope, and may be ufed with good effect for 

 magnifying fmall tranfparent objects ; but in general 

 it is applied to the purpofe of amufement,- by calling 

 the fpecies or image of a fmall tranfparent painting on 

 glafs upon a white wall or fcreen, at a focal diftance 

 from the inftrument. 



Let a candle or lamp C (fig. 47.) be placed in the 

 infide of a box, fo that the light may pafs through the 

 plano-convex tens N N, and ftrongly illuminate the 

 object OB, which is a tranfparent painting on glafs, in- 

 vert jd and moveable before N N, by means of a flicj- 



* Fergufou's Le&ures, p. 235. 

 f Sec Phil. Tranf. 1784. 



ing 



