Dr Goring on Reflecting Microscopes, 36 



be perfectly in unison with its curve or figure, and likewise 

 with the tube. Thus, in the metal mentioned, three-tenths fo- 

 cus, and two-tenths aperture, the thickness of the metal and sup- 

 port at the back is not usually less than two-tenths of an inch. 

 Of all these difficulties in execution, a mere theorist in the closet 

 must of course be supposed ignorant ; but j/" it is absolutely 

 necessary to make these small metals very thick, and I think it 

 will be found so, how can the two metals in Sir David Brew- 

 ster's construction be sufficiently approximated to each other, 

 and the size of the plane increased in the requisite proportion 

 necessary to enable the instrument to act on an opaque object 

 with its full aperture, without reducing the visual pencil to a 

 mere ring of light, producing, in consequence, a strong nebulosity 

 in the middle of the field of view, and making the instrument as 

 dark as a wolf's mouth. 



The metal of three-tenths focus, and two-tenths aperture in 

 the Amician form, acts perfectly well upon opaque objects, 

 either with or without a small silver cup. I do not believe 

 that an opaque object will ever be shewn at all by a similar 

 speculum in Sir David Brewster's. 



Athly, It must be evident, that a tube presented sideways to- 

 wards an object, will allow the Light to fall on an opaque body 

 placed very near to it ; whereas, the back of a metal of the same 

 diameter with the tube, placed at the same distance from the 

 object, will completely shade it. 



5thly, A plane metal reflecting at right angles as in the 

 Brewsterian form, can by no means reverberate so much light 

 as one at an angle of 45°, and it is this circumstance which gives 

 the superiority to the Newtonian telescope over the Cassegrai- 

 nian and Gregorian forms. 



In a note appended to Sir D. Brewster's critique on my solar 

 camera microscope, he is pleased to quiz me rather unmercifully 

 about my ignorance of the fact, that total beflection takes 

 place at an angle of 41° 49' from the surfaces of glass ! We 

 must all live and learn. I shall proceed, therefore, to turn this 

 grand and capital discovery to account. First, then, I shall 

 substitute a bit of plane glass for the diagonal metal of the 

 Amician, (which latter certainly does by no means, any more 

 than a silvered rectangular prism, reflect all the light which 



