Dr Goring on Reflecting Microscopes. 89 



pose the concave metal to be enclosed in a frame v^hich is to be 

 fitted into the tube with a bayonet catch, that it may c^^^f£ 

 in exactly into the same relative position ; to the back of this 

 frame is to be soldered a tube made truly centrical with it, o 

 receive the eye piece, which is to be passed into it, so as to 

 project four inches beyond the surface of the parabohc metal ^n. 

 to 'the body of the telescope ; it must be always inserted exactly 

 in the same place relative to its containing tube, and a mark 

 must be made on each which must be made to correspond (as 

 in the case of the eye glasses), for fear of derangzng the eye- 

 hole, which will always be dehcate and ticklish in its operation 

 in this telescope, being only guarded by the image of the nar 

 row black circlet which encompasses the small metal. It the 

 said circlet should after all be found inefficient, it may be made 

 one-sixteenth of an inch wider, so as to make the entire diame- 

 ter of the small metal one inch. 



Now, let us observe the manner in which the primary image, 

 formed by the concave speculum, is reached by the eyepiece. 

 The small metal, the surface of which, placed at 1^ inch dis- 

 tance from the end of the cone of rays, reverberated by the 

 concave speculum, reflects its image to within 5i inches dis- 

 tance from the hole in it ;-the tube of the eyepiece^rojects 

 4 inches, and its focus is li inch, li -h 1^ + 4 + U -9, the 

 whole distance to be got over. 



Now, if I am asked whether a Gregorian of the same power 

 and aperture would not be superior to this instrument? 1 

 answer, that it would be superior as to hght with any given 

 power, in the ratio of what is stopped by the achromatic glass 

 or glasses; it will also be without any defects in performance 

 which may be occasioned by it or them. Still, however, I con- 

 sider that the instrument just described will have abundance of 

 light with all the power I propose to apply to it, and that the 

 eye will never be able to appreciate any difference m perform- 

 ance on terrestrial objects produced by the achromatics (i\ 

 well executed). But,in compensation, the Gregorian construction 

 labours under two great disadvantages ; the first is, it can only 

 be workedperfect to one power, for immediately you alter this one 

 power (no matter how), the instrument begins to aberrate. Ihe 

 second is, that when made dumpy, the difficulty of working its 



