330 Account of Mr Prilchard's Single Lens Microscopes 



indeed has himself found some of the diamond lenses which 

 he made, quite defective, giving something like a treble image. 

 Hence it would be adviseable to manufacture diamond lenses 

 only out of plates of diamond, through which it is easy to ex- 

 amine by polarized light its doubly refracting structure, 

 and to reject all the plates in which there is the slightest ten- 

 dency to this structure. If the diamond has not sufficiently 

 flat surfaces to admit of this experiment being easily made, it 

 should be examined when immersed in oil of cassia or sulphu- 

 ret of carbon, — the fluids which approach nearest to it in re- 

 fractive power. 



Mr Pritchard mentions, that he has also "• formed lenses of 

 the other precious stones, but without any peculiar advantage, 

 many of them producing two magnified images by double 

 refraction." Zircon^ Essonite, JEuclase, and some others, 

 would no doubt produce this effect to a great degree ; but 

 Garnet, Pyrope, Spinelle, and Rub?/ will not give double 

 images. We have examined specimens of garnet, &c. so per- 

 fectly pure, that we would recommend strongly to Mr Prit- 

 chard to devote his attention to this substance. Its refrac- 

 tive index is 1.815 greater than that of sapphire, while its 

 power of dispersion is 0.33 inferior to 'diamond, so that, from 

 its having no double refraction, it unites the theoretical requi- 

 sites for a perfect microscope in a greater degree than either 

 diamond or sapphire. As the observation of colour is the 

 least of all considerations, and is besides a very fallacious one 

 in microscopic observations, the colour of the garnet cannot 

 be regarded as a disadvantage. It is on the contrary an ad- 

 vantage, as it renders the microscope more achromatic by its 

 absorption of the violet or most refrangible rays. But even 

 if the observation of colour were material, we can determine 

 it as clearly by a coloured as by a colourless lens. If, for ex- 

 ample, the garnet lens shows an object or part of an object of 

 a certain apparent colour, it is easy to determine the real co- 

 lour by ascertaining what colour seen through the lens pro- 

 duces the apparent colour under consideration. 



As it is now perfectly easy to illuminate microscopic ob- 

 jects with homogeneous light, we may set aside all considera- 

 tion of the dispersive power of bodies, and employ for single 



