356 OBJECTIVES, EYE-PIECES, THE APERTOMETER 



from the radiant point, /, h e being a normal to the convex 

 surface, and i d to the plane one under these circumstances the 

 angle of emergence, y e h, much exceeds that of incidence, f d i, 

 being probably almost three times as great. 



If the radiant is now made to approach the glass, so that the 

 course of the ray, fd e g, shall be more divergent from the axis, as 

 the angles of incidence and emergence become more nearly equal 

 to each other, the spherical aberration produced by the two will be 

 found to bear a less proportion to the opposing error of the single 

 correcting curve a c b ; for such a focus, therefore, the rays will be 

 over-corrected. But if / still approaches the glass, the angle of 

 incidence continues to increase with the increasing divergence of 

 the ray, till it will exceed that of emergence, which has in the mean- 

 while been diminishing, and at length the spherical error produced 

 by them will recover its original proportion to the opposite error of 

 the curve of correction. When f has reached this point f (at which 

 the angle of incidence does not exceed that of emergence so much as 

 it had at first come short of it), the rays again pass the glass free 

 from spherical aberration. 



If/ be carried hence towards the glass, or outwards from its 

 original place, the angle of incidence in the former case, or of 

 emergence in the latter, becomes disproportionately effective, and 

 either way the aberration exceeds the correction. 



How far Lister's discoveries were affected by Amici's work it is 

 now quite impossible to say ; there can be but little doubt that some 

 influence is clue to it, but it is equally clear that a profound know- 

 ledge of the optics of that time was the only foundation upon which 

 the facts in Lister's paper could have been built. He was a man of 

 application and an enthusiast, and it was inevitable that he should 

 exert a powerful influence upon the early history of the optics of the 

 microscope. This is the more certain when we remember how few 

 were the men at that time who knew in any practical sense what a 

 microscope was ; and we find that in 1831, being unable to find any 

 optician who cared to experiment sufficiently, Lister taught himself 

 the art of lens-grinding, and he made an objective whose front was 

 a meniscus pair, with a triple middle combination, and the back a 

 plano-convex doublet. He declared this to be the best lens of its 

 immediate time, and it had a working distance of !!. 



One of the immediate consequences of the publication of Lister's 

 paper was the rapid production by professional opticians of achromatic 

 objectives. The data supplied by Lister proved to be of the highest 

 value in the actual production of these, and the progress of improve- 

 ment was, in consequence, and in comparison with the time imme- 

 diately preceding, remarkably rapid. 



Andreiv Ross began their manufacture in 1831. He was followed 

 T>y Hugh Powell in 1834, and in 1839 by James Smith. It is of 

 more than ordinary interest to study in detail the work of this im- 

 mediate time, and the following table giving a list of objectives, with 

 their foci, apertures, and mode of construction, with the dates of 

 their production, will give a fair idea of the work of Andrew Ross 

 in the manufacture of early lenses. He was the earliest of the three 



