Joseph Lister

“ Each of Chevalier's compound plano-convex lenses when used singly presented a bur or coma outwards, but when two of them are combined this coma, instead of being exaggerated, as might have been expected, was ‘less than with any single glass,’ while the performance was in other respects satisfactory. ‘Observing the advantages resulting from this combination,’ Lister ‘tried some others,’ among the rest two of Tulley's triple glasses, each of which taken singly was of fine performance. ”

“ In 1837 A. Ross made an unsuccessful experiment with ‘three glasses to admit a large pencil.’ Lister thereupon suggested a combination of three glasses ‘for the same object;’ he gave the dimensions of the lenses and the curves of the various surfaces, with a statement of the effect proposed to be produced by each glass upon spherical aberration and coma. This resulted in Ross's celebrated 1⁄8 inch object-glass, the construction of which was afterwards adopted by the other principal London makers. ”

“ On the other hand, separation of the glasses beyond their original interval produces under-correction. Thus, by merely varying the distance between two such lenses, the correction of the spherical error may be either increased or diminished at pleasure according to a definite rule, and slight defects in the glasses can be remedied by simply altering their relative position, the achromatism of the combination being meanwhile little affected.
Lister also explained the relation of the aplanatic foci to the coma. At the shorter focus the coma is inwards, at the longer focus it is outwards ”