achromatic compound Microscope. 1 79 



is much augmented ; as in an object-glass placed in front of 

 another : but, generally, in this position a part only of its 

 aperture comes into use ; so that the two properties mention- 

 ed neutralize each other, and its chromatic state remains unal- 

 tered. If, for example, the outstanding colours were observ- 

 ed at the longer focus to be green and claret, which show that 

 the nearest practicable 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 ob- 

 ject-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 colour 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 conver- 

 gence, and the shorter focus has borne a rather less propor- 

 tion to the longer. 



If the inner surfaces are not cemented, a striking effect is 

 produced by minute differences in their curves. It may give 

 some idea of this, that in a glass of which almost the whole 

 disk was covered with colour from contact of the lenses, the 

 addition of a film of varnish so thin that this colour was not 

 destroyed by it, caused a sensible change in the spherical cor- 

 rection. 



. I have found that whatever extended the longer aplantic 

 focus, and increased the convergence of its rays, diminished 

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

 concave 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. 



The direction of the aplanatic pencils appears to be scarce- 



