A HISTORY OF SCIENCE 



lenses, and seemed all but insuperable. The making 

 of achromatic lenses for telescopes had been accom- 

 plished, it is true, by Dolland in the previous century, 

 by the union of lenses of crown glass with those 

 of flint glass, these two materials having different 

 indices of refraction and dispersion. But, aside from 

 the mechanical difficulties which arise when the lens is 

 of the minute dimensions required for use with the mi- 

 croscope, other perplexities are introduced by the fact 

 that the use of a wide pencil of light is a desideratum, 

 in order to gain sufficient illumination when large mag- 

 nification is to be secured. 



In the attempt to overcome these difficulties, the 

 foremost physical philosophers of the time came to the 

 aid of the best opticians. Very early in the century, 

 Dr. (afterwards Sir David) Brewster, the renowned 

 Scotch physicist, suggested that certain advantages 

 might accrue from the use of such gems as have high 

 refractive and low dispersive indices, in place of lenses 

 made of glass. Accordingly lenses were made of dia- 

 mond, of sapphire, and so on, and with some measure of 

 success. But in 1812 a much more important inno- 

 vation was introduced by Dr. William Hyde Wollas- 

 ton, one of the greatest and most versatile, and, since 

 the death of Cavendish, by far the most eccentric of 

 English natural philosophers. This was the sugges- 

 tion to use two plano-convex lenses, placed at a pre- 

 scribed distance apart, in lieu of the single double-con- 

 vex lens generally used. This combination largely 

 overcame the spherical aberration, and it gained im- 

 mediate fame as the " Wollaston doublet." 



To obviate loss of light in such a doublet from in- 



ITO 



