328 Account of Mr Pritchard's Single Lens Microscopes 



Number. In the meantime, we shall proceed to give an ac- 

 count of the single microscopes of sapphire and diamond, which 

 have been so successfully executed by Mr Pritchard. 



In the years 1810 and 1811, when Dr Brewster had deter- 

 mined the refractive and dispersive power of the gems, and 

 found that some of them united very low dispersive with very 

 high refractive powers, he pointed out the advantages of such 

 an union of optical properties, for the construction of single mi- 

 croscopes. About ten years ago, Mr Peter Hill, an ingenious 

 optician in Edinburgh, executed for him two single lenses of 

 rubi/ and garnet, which were used both as single microscopes, 

 and as the object-glasses of a compound microscope. Mr Siv- 

 right of Meggetland had also executed for him, we believe by the 

 same artist, a single piano convex lens, of the colourless topaz 

 of New Holland. Such were the attempts which had been made 

 previous to the labours of Mr Pritchard, who has given the 

 following account of them in the Treatise on Optical Instru- 

 ments, published by the Society for the diffusion of Knowledge. 



" Dr Brewster, in his Treatise on New Philosophical Instru- 

 ments, speaking of single microscopes, says, — ' We cannot 

 expect any essential improvement in that instrument, unless 

 from the discovery of some transparent substance, which, like 

 the diamond, combines a high refractive power with a low 

 power of dispersion.' This substance has subsequently been 

 formed into lenses by Mr A. Pritchard, at the suggestion of 

 Dr Goring, who caused Mr P. to commence the undertaking 

 in June 1824. The first diamond lens was completed at the 

 end of that year. The difficulty of working this substance 

 into a perfect figure was subsequently overcome. Mr Pritchard 

 finished the first diamond microscope in 1826; the focal dis- 

 tance of this magnifier, which is double convex, is about j^o^^ 

 of an inch. Of the value and importance of the introduction 

 of this brilliant substance for the formation of single micro- 

 scopes, Dr Goring states, " I conceive diamond lenses to consti- 

 tute the ultimatum of perfection in the single microscope. 



*' The principal advantages of employing this brilliant sub- 

 stance in the formation of microscopes, arise from the natural- 

 ly high refractive power it possesses, whereby we can obtain 

 lenses of any degree of magnifying power, and that with com- 



