198 Experimental Philosophy. [Lecture IS. 



To estimate the magnifying power of the Gre- 

 gorian telescope, multiply the focal distance of the 

 large mirror by the distance of the small mirror 

 from the image S ; then multiply the focal dis- 

 tance of the small mirror by the focal distance of 

 the eye-glass Tcr ; lastly divide these two products 

 by one another, and the quotient is the magnify- 

 ing power. 



Sir Isaac Newton formed his telescope upon 

 a somewhat different principle from that of 

 Gregory. In his instrument, still known by the 

 name of the Newtonian telescope, instead of the 

 small concave mirror en, there is placed diago- 

 nally a plane mirror, on which the spectator looks 

 through the side of the telescope by means of an 

 eye-glass adapted to that purpose. The cele- 

 brated Dr. Herschel commonly uses the New- 

 tonian telescope on an improved principle, and 

 through that makes most of his observations. 



Dr. HerschePs great telescope is however of a 

 different construction. It has only one large con- 

 cave reflector at the bottom of the tube ; and the 

 spectator stands with his back to the object, and 

 looks in upon the reflector through an eye-glass. 

 The magnifying power of this is the same as that 

 of a Newtonian telescope would be of the same 

 sized reflector; but, there being only one re- 

 flector, the quantity of light is less diminished. 

 A minute description of this curious telescope is 

 given under the word TELESCOPE in that uni- 

 versal dictionary called the Pantologia* 



