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; and in a combination of two lenses arranged as above described, the inward coma from the shorter focus of the front glass destroys the outward coma from the longer focus of the back glass, and ‘the whole field is rendered beautifully flat and distinct.’
The same principle applies when the lenses are of different form, and when more than two are combined. Thus Lister reduced the manufacture of the achromatic object-glass from a matter of uncertainty and empiricism to a scientific system, and it has become susceptible of a degree of perfection that would otherwise have been impossible.
But Lister continued his labours after he had discovered the principle of construction. A section of his notes is labelled ‘Memoranda on object-glasses made for experiment, December 1829 to May 1830,’ including highly interesting accounts of the effects of glasses made by his own hands. He wrote to Sir John Herschel on 24 Feb. 1831: ‘Finding, however, that W. Tulley was too busy to pursue for me the experiments I wished for ascertaining how compound object-glasses could be combined to the greatest advantage, I determined in November last to make a trial myself. The result was, I acknowledge, beyond my expectations; for without having ever before cut brass or ground more than a single surface of a piece of glass, I managed to make the tools and to manufacture a combination of three double object-glasses, without spoiling a lens or altering a curve, which fulfilled all the conditions I had proposed for a pencil of thirty-six degrees.’ … ‘About three weeks ago I made a second and more complicated trial projected for obtaining the same effect with a much larger pencil. This is just finished, but not without altering one of the original curves, and its plan might be improved if I could spare time to make another set. Still I flatter myself these attempts would interest thee, as showing how easily the principle I mastered may enable an utter novice in glass-working to produce vision which I have not yet seen exceeded.’ In the second of these trials he deviated from the plano-convex form of the lenses, employing a combination of three, of which the front was a double meniscus, the middle a triple, and the back one a double plano-convex. The reasons for preferring these forms are given in full detail in his notes, among which occurs the ingenious idea of regarding the triple with the middle of flint glass as divided by an imaginary line through the flint into two double achromatic glasses, each of which may be considered separately as having two aplanatic foci. The object he proposed to himself was ‘a construction fitted to obtain the largest pencil with good front space and without coma;’ and after describing the mode by which this was arrived at, he says: ‘This combination proves most satisfactorily the advantage of keeping the angles of the rays at all the different curves moderate, the vision being singularly definite and easy. … Indeed, taking all together, I think I have met with nothing to equal it, the distance of the front glass from the object being 0.11 full.’
Having now completely satisfied himself of the applicability of his principle, he devoted much of his leisure for several years to various investigations by aid of the instrument which he had so greatly improved. He thus brought to light many new facts regarding the structure of the animal body. He was the first to ascertain the true form of the red corpuscle of mammalian blood, and selections from his observations on zoophytes and ascidians, beautifully illustrated by sketches from life by the camera lucida, form a classical paper in the ‘Philosophical Transactions,’ 1834. A laborious inquiry, chiefly conducted by means of the microscope, into the limits of human vision, as determined by the nature of light and of the eye, has not been published. He had prepared an account of it for the press, and was on the eve of publication when he learned that the astronomer royal, Professor Airy, had reached the same conclusions, though by a different road, and so abandoned the idea.
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 ⅛ inch object-glass, the construction of which was afterwards adopted by the other principal London makers.
For many years after this date Lister continued to aid the opticians in the construction of the microscope. He died on 24 Oct. 1869, in the eighty-fourth year of his age.
Various improvements have been since introduced both in this country and abroad in the construction of the achromatic object-glass; but Lister's law of the aplanatic foci remains the guiding principle as ‘the pillar
