ZOOLOftY AND BOTANYj MICEOSCOPY, ETC. 297 



One arises from the unequal course of the dispersion in crown- 

 glass and flint-glass, in consequence of which it is impossible with the 

 present kinds of glass, to unite perfectly all the coloured rays in the 

 image. In the best combinations of lenses which can be made, there 

 is always therefore a considerable secondary chromatic aberration in 

 the image which impairs its clearness. 



The second still greater hindrance is the inequality of the spherical 

 aberration of an objective for light of different colours, and the im- 

 possibility of compensating this inequality with our present resources. 

 It is not difficult even with a large aperture (using light of one par- 

 ticular degree of refrangibility) to remove perfectly, practically 

 speaking, the spherical aberration, at least in the axis, so that the 

 objective, with monochromatic light of this fixed colour, will give an 

 almost perfect union of the rays; the system is however under- 

 corrected for the less refrangible rays, and over-corrected for the 

 stronger. The larger the aperture of an objective, the greater of course 

 will be the residual aberration which originates in this difference of 

 the spherical correction for the various colours. Their effect appears 

 in the form of a characteristic diversity which the chromatic correction 

 of the objective shows for the different zones of the free aperture, au 

 objective which possesses the most perfect possible chromatic cor- 

 rection for the central rays, and gives the most favourable images 

 with direct illumination, being more or less strongly over-corrected 

 chromatically for the peripheral rays, and with oblique illumination 

 shows the outline of the object with distinct chromatic fringes, and 

 conversely. In objectives of moderate aperture, perhaps up to 40^ 

 or 50°, we may restrict the detrimental effect of this chromatic 

 difference of the spherical aberration, by dividing the refractions 

 over a greater number of separate lenses than would be otherwise 

 required by the aperture. English and American opticians have in 

 this way constructed weak objectives of from 30 to 20 mm. focal 

 length, which give a more perfect union of the rays than the corre- 

 sponding more simply constructed lenses in use on the Continent, 

 and which allow of a much higher magnifying power by means of 

 draw-tube and eye-piece. The above-mentioned class of aberrations 

 offer, however, an insurmountable difficulty in the case of the large 

 apertures of dry and immersion objectives. The impossibility of 

 removing them entirely with the present resources must be un- 

 questionably considered the greatest difficulty which has hitherto 

 hindered a more perfect action of the objective, with regard to its 

 dioptrical working. 



It is not difficult to define the cause from which this defect 

 springs. The impossibility of removing the chromatic difference of 

 spherical aberration, originates in the fact that with the existing kinds 

 of glass (crown-glass and flint-glass), the dispersion increases with the 

 mean refractive, index in such a manner that greater dispersion always 

 accompanies the higher index (with very slight deviations) and con- 

 versely. The aberrations could be compensated for, or at least 

 nearly so, if there were materials applicable to optical purposes, by 

 which a relatively smaller refractive index could be united with 



Ser. 2.— YoL. IV. X 



