62 Dr Goring on Monochromatic Light, 4"C 



precisely, at the lens Cg, but about f \tb of an inch nearer the 

 eye-glass. 



" It is obvious that this combination of glasses may be used 

 as a microscope ; for if, instead of the image formed by the 

 object-glass at FG, we substitute a small object, illuminated 

 from behind, as in compound microscopes ; and if we draw 

 the eye-piece a very small way from this object, the pencils of 

 parallel rays emergent from the eye-glass D, will become con- 

 vergent to very distant points, and will there form an in- 

 verted and enlarged picture of the object, which may be viewed 

 by a Huygenian eye-piece ; and we may thus get high magni- 

 fying powers without using very deep glasses. We tried the 

 eye-piece, of which we have given the dimensions, in this way, 

 and found that it might be made to magnify 180 times with 

 very great distinctness. When used as the magnifier of a so- 

 lar microscope, it infinitely surpasses every thing we have ever 

 seen. The picture formed by a solar microscope is generally 

 so indistinct, that it is fit only for amusing ladies ; but with 

 this magnifier it seemed perfectly sharp. We therefore recom- 

 mend this to the artists as a valuable article of their trade.' 1 '' 



I caused that distinguished artist, Mr Adie, of Edinburgh, 

 to execute for me a microscope similar to that recommended 

 by the Professor, on which no expence was spared ; it of 

 course gave an erect image. I was not content with it, its 

 construction being too complicated, and its length inconve- 

 nient ; moreover, though its angle of aperture was no greater 

 than that usually given to eye-pieces, its chromatic aberration 

 forced itself occasionally on my unwilling conviction in a man- 

 ner which I could not resist in spite of my faith in analysis. 

 I naturally, therefore, imputed this defect to Mr Adie's im- 

 perfect execution, and set him to work afresh to make another 

 microscope on the plan of an erecting eye-piece of the ordi- 

 nary construction, gaining the different powers by the appli- 

 cation of Huygenian eye-pieces to the image formed by the 

 two bottom glasses. I was very much pleased by the distinct- 

 ness of this microscope, and so were all who examined it. Still, 

 however, it was not achromatic, so I set Mr Adie down, at the 

 time, for a bungler who could not adjust the foci and intervals 

 of the glasses in a proper manner; and determined to have 



