32 Dr Goring on Eeflecting Microscopes. 



in an ordinary reflecting telescope, and looks very weU. 



Mak- 



ing improvements on optical instruments on paper is a delight- 

 ful amusement ; but it is something like raising troops and re- 

 venues in the same way. However, when the drawings are trnly 

 made upon rather a large scale, it cannot be denied that much 

 may be demonstrated by them, so that the cost and trouble of exe- 

 cuting things, which cannot possibly turn out to be of any real 

 utility in a practical point of view, may thus frequently be 

 saved. I have appended to this paper a tolerably true draw- 

 ing of a metal of 27|° of acting aperture ; it may be supposed 

 to be any focal length, for example six-tenths of an inch. The 

 Brewsterian and Amician constructions are both shewn in it, 

 and speak for themselves as to the portion of the concave me- 

 tal which their plane specula respectively obliterate and render 

 inert. A A, is the concave ; B, Sir David's plane mirror ; C, 

 Amici's, [both viade of the smallest size pos- 

 sible, their extreme edges being in fidl ope- 

 ratioJi, and the object supposed to be close 

 to the back of the concave, in the one case 

 at E; and to the side of the tube in the 

 other at D :) F F, are sections of the sides 

 of the tube ; G G, sections of the piece of 

 brass applied to the back of the concave to 

 fasten it in its place ; e, is the focus of the 

 concave, supposed not tohave been reflected 

 back by the plane B to E, or by the diagonal C 

 to D ; the four lis are dotted lines to express 

 the alignments of the diagonal metal and the 

 large plane with the conmion elliptical one. 

 It will be found by admeasurement that the large plane blots 

 out one-half o{ the centrical and finest part of the concave, and, 

 consequently, one-quarter of the whole of its light ! The dia- 

 gonal little more than one-quarter of the centrical portion of 

 metal, and, consequently, not much more than one-sixteenth of 

 the light. This is according to the drawing, which is made 

 exactly as fair for one construction as for the other. In prac- 

 tice, howevex-, it will be found that as it is necessary to cause 

 the pencil to be projected outwards considerably more than in 

 the said drawing^or the purpose of shewing opaque objects, both 



