36 Dr Goring on Beflecting Microscopes. 



fallsuponit). Total reflection will then take place from the 

 diagonal; now, then, is Sir David's instrument beaten hollow, 

 as I suppose he will admit. His double achromatic prism for 

 the Newtonian telescope, and that for the Amician engiscope, 

 to refract the rays outside of the tube, instead of using a plane 

 diagonal, must be laid on the shelf ; a bit of plane flint glass 

 is a far better thing ; from the Newtonian telescope down to a 

 camei'a obscura, let glass be substituted for diagonal metals, 

 looking-glass, and rectangular prisms silvered on the reflecting 

 side, — let the Herschelian telescope be abandoned for the New- 

 tonian, with a glass diagonal, which will be just as luminous ; 

 for the head of the observer in the former will intercept as 

 much light as the plane piece of glass. 



Lastly., if people are not satisfied of the validity of my objec- 

 tions to Sir David's instrument, — I say let itbe executed, and tried 

 against an Amician. 



Sir David has recommended the combination of metals, whoso 

 merits I have just discussed, to form a solar reflecting mi- 

 croscope, for which purpose, the principle is still less adapted 

 than for an engiscope. He, moreover, seems to think a perforat- 

 ed metal with an opaque object initsfocus, illuminated through 

 the hole behind it (which was invented by Mr C. Tulley v. " Mi- 

 crographia,'' p. 40), will make a most capital reflecting solar for 

 opaque objects, but I can only say it never did so in my hands. ■ 



Itismuch to be regretted, thatSir D. Brewster has not favoured 

 us mth an account of the celebrated solar microscope, which / 

 have been told, was executed under his direction, I think, for 

 the University of Edinburgh, and which gave such universal 

 satisfaction. This is surely a great sin of omission in his trea- 

 tise on the microscope, where I had fully expected to have met 

 with it. I cannot help thinking that, when Sir J. Newton in- 

 vented his telescope, the idea of causing the cone of rays to be 

 returned back by a plane speculum to a hole in the centre of 

 the concave metal must have presented itself to his mind, or to 

 any man's who thought on the subject, especially as the Gre- 



■* This seemed to arise from the ilhimination being too direct and ghhig no 

 ihades ; for the same reason, I suppose, I never succeeded in getting a good 

 image of an'oi)aque object in the solar from a leus set in a silver cup ; the 

 iUuminatlon must le oblique, or all is confusion. 



