THE MICROSCOPE. 27 



glass which is achromatic, with a moderate aperture, has it 

 cell opened wider, the circle of rays thus added to the penci 

 will be rather over-corrected as to color. 



" The same tendency to over-correction is produced, if, with- 

 out varying the aperture, the divergence of the incident rays is 

 much augmented, as in an object-glass placed in front of an- 

 other; but generally in this position a part only of its aperture 

 comes into use; so that the two properties mentioned neutral- 

 ize each other, and its chromatic state remains unaltered. If, 

 for example, the outstanding colors were observed at the longer 

 focus to be green and claret, which show that the nearest prac- 

 ticable approach is made to the union of the spectrum, they 

 usually continue nearly the same for the whole space between 

 the foci, and for some distance beyond them either way. 



"The places of these two foci and their proportions to each 

 other depend on a variety of circumstances. In several object- 

 glasses that I have had made for trial, plano-convex, with their 

 inner surfaces cemented, their diameters the radius of the flint 

 lens, and their color pretty well corrected, those composed of 

 dense flint and light plate have had the rays from the longer 

 focus emerging nearly parallel; and this focus has been not 

 quite three times the distance of the shorter from the glass: 

 with English flint the rays have had more convergence, and the 

 shorter focus has borne a rather less proportion to the longer. 



"If the surfaces are not cemented, a striking effect is pro- 

 duced by minute differences in their curves. It may give some 

 idea of this, that in a glass of which nearly the whole disk was 

 covered with color from contact of the lenses, the addition 

 of a film of varnish, so thin that this color was not destroyed 

 by it, caused a sensible change in the spherical correction. 



"I have found that whatever extended the longer aplanatic 

 focus, and increased the convergence of its rays, diminished 

 the relative length of the shorter. Thus by turning to the con- 

 cave lens the flatter instead of the deeper side of a convex 

 lens, whose radii were to each other as 31 to 35, the pencil of 

 the longer aplanatic focus, from being greatly divergent, was 

 brought to converge at a very small distance behind the glass; 

 and the length of the shorter focus, which had been one-half 

 that of the longer, became but one-sixth of it. 



