198 MICROSCOPIC OBJECTS. 



The idea has often occurred to me, that as flint-glass 

 is now made on the Continent of a very high specific 

 gravity, and at the same time free from veins and of the 

 most perfect clearness and transparency, it might be 

 employed in making the eye-pieces of both telescopes 

 arid microscopes, and would have the effect of increasing 

 the size and bettering the edges of the field of view, 

 somewhat after the manner that doubling the glasses 

 does — ^that is, by reducing the spherical aberration of 

 the edges from the shallower curvature they would 

 require for any given power, ivhile the achromatism of 

 negative eye-pieces could always he preserved as usual. 

 But it must not be supposed that I recommend this for 

 positive eye-pieces, or single ones of any kind, as the 

 high chromatic dispersion of such flint-glass could not in 

 these be corrected, and would neutralize the advantages 

 procured by diminishing the spherical aberration. Neither 

 can I be answerable that the advantages obtained by 

 using flint- instead of plate-glass would be sufficiently 

 sensible to be appreciated by the eye. I intend, however, 

 to have the experiment tried. I have a plano-convex 

 lens of half an inch focus, made of the dense flint-glass of 

 Guinand, which I think gives a sensibly larger and better 

 field of view than one of equivalent focus and the same 

 figure made of plate-glass, which latter of course has a 

 deeper curve. It is certain that it ought to do so 

 according to theory. 



I had nearly forgotten to mention that the shallow 

 object-glass at e, when used alone, must always remain 



