ACHBOMATiSM. 
7 
don, by combining a double convex lens of crown glass with a 
concave lens of flint glass, succeeded in constructing an object- 
glass for the telescope in which the greater dispersive power of 
the flint glass served to correct the chromatic aberration of the 
convex lens of crown glass, while a considerable portion of the 
refractive power of the crown glass remained as the efficient 
power of the compound lens. This principle was soon thor- 
oughly established as in every way successful for forming achro- 
matic lenses for the telescope. Efforts were made in the early 
part of the present century to improve the microscope in the 
same manner. In applying this principle to the microscope 
several difficulties were encountered. First, it was found 
extremely difficult to work the curves of such small lenses with 
sufficient accuracy to insure freedom from chromatic aberra- 
tion. Secondly, while lenses for the telescope of large diame- 
ter constituted but very small segments of spheres, the lenses 
requiring long foci, and being adapted to receive rays very 
nearly parallel, lenses for the microscope, receiving light radia- 
ting from a point very near the lens, when made of compara- 
tively moderate diameter constituted very much larger seg- 
ments of spheres than the lenses of telescopes. Thirdly, it was 
found that when the aberrations of a convex lens of crown 
glass were corrected by a concave lens of flint glass, if the 
diameter of the lens was enlarged beyond very moderate limits, 
the correction for the borders of the lens became too great in 
proportion to the central portion, so that there seemed to be a 
limit, and that a very small one, to the available aperture of 
achromatic lenses constructed on this principle. So great, and 
apparently invincible, were these difficulties, that as late as 
1824:, such philosophers as Biot and Wollaston predicted that 
the compound achromatic microscope could never be brought 
to the same degree of perfection as the achromatic telescope. 
14:. Combinetl I^enses. To overcome these difficulties, dif- 
ferent opticians combined two or more compound lenses, each 
made separately as nearly achromatic as possible. By this 
means a higher magnifying power was obtained, and a larger 
angular pencil of light was transmitted, though considerable 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
