262 MICROSCOPE glass with convex lenses of crown, while their spherical aberration is corrected by the combination (as in Herschel s aplanatic doublet) of convex and concave surfaces of different curvatures. The minute size and high curvature of the lenses required as microscopic objectives were long considered as altogether precluding the possibility of success in the production of such combinations, more especially as the conditions they would have to meet differ altogether from those under which telescopic object-glasses are employed. For the rays from distant objects fall upon the latter with virtual parallelism ; and the higher the power required the longer is the focus given to them, and the smaller is the deflexion of the rays. In the microscope, on the other hand, the object is so closely approximated to the objective that the rays which proceed to it from the latter have always a very considerable divergence ; and the deflexion to which they are subjected increases with that reduction of the focal length of the objective which is the necessary condition of the increase of its magnifying power. And thus, although the telescopic " triplet " worked out by Dollond (consisting of a double-concave of flint glass, interposed between two double-convex lenses of crown) can be so constructed as to be not only completely aplanatic (or free from spherical aberration) but almost completely achromatic (or free from chromatic aberration), this construction is only suitable for microscopic objectives of long focus and small angular aperture, the rays falling on which have but a very moderate divergence. And though, as will presently appear, some of the early attempts at the achromatization of the microscope were made in this direction, it was soon abandoned for other plans of construction, which were found to be alike theoretically and practically superior. It seems to have been by Professor Amici, then of Modena, about 1812, that the first attempts were made at the achromatization of microscopic objectives ; but, these attempts not proving successful, he turned his attention to the production of a reflecting microscope, which was a decided improvement upon the non-achromatized compound microscopes then in use. In the year 1820, however, the subject was taken up by Selligues and Chevalier of Paris, who adopted the plan of superposing three or four com binations, each consisting of a double-convex of crown cemented to a plano-concave of flint. The back combina tion (that nearest to the eye) was of somewhat lower power than those placed in front of it, but these last were all of the same focus, and no attempt was made by these opticians to vary the construction of the several pairs thus united, so as to make them correct each others aberrations. Hence, although a considerable magnifying power could be thus obtained, with an almost complete extinction of chromatic aberration, the aperture of these objectives could not be greatly widened without the impairment of the distinctness of the image by a "coma" proceeding from uncorrected spherical aberration. In ignorance, it would appear, of what was being done by the Paris opticians, and at the instigation of Dr Goring (a scientific amateur), Mr Tulley well known in London as an able constructor of telescopic objectives began, about the year 1824, to work object-glasses for the micro scope on the telescopic plan. After many trials 1 he succeeded, in 1825, in producing a triplet of -$ inch focus, admitting a pencil of 18, which was so well corrected as to perform very satisfactorily with an eye-piece giving a magnifying power of 120 diameters. He afterwards made a similar triplet of shorter focus, which, when placed in 1 It is due to Mr Joseph J. Lister to mention that Tulley s final success with this low power seems to have been attained by working on a suggestion given him by that gentleman. See Monthly Micro scopical Journal, vol. iii. (1870), p. 134. front of the previous one, increased the angle of the trans mitted pencil to 38, and bore an eye-piece giving a magnifying power of 300 diameters. These triplets are said by Mr Ross to have never been exceeded by any similar combinations for accurate correction throughout the field. Having come into possession, at the end of 1826, of an objective of Chevalier s construction, Mr J. J. Lister carefully examined its properties, and compared them with those of Tulley s triplets ; and this comparison having led him to institute further experiments he obtained results which were at first so conflicting that they must have proved utterly bewildering to a less acute mind, 2 but which finally led him to the enunciation of the principle on which all the best microscopic objectives are now constructed. For he discovered that the performance of such com posite objectives greatly depends upon the relative position of their component combinations, the effect of the flint plano-concave upon the spherical aberration produced by the double-convex of crown varying remarkably according to the distance of the luminous point from the front of the objective. If the radiant is at a considerable distance, the rays proceeding from it have their spherical error under- corrected ; but, as the source of light is brought nearer to the glass, the flint lens produces greater proportionate effect, and the under-correction diminishes, until at length a point is reached where it disappears entirely, the rays being all brought to one point at the conjugate focus of the lens. This, then, is one aplanatic focus. If, however, the luminous point is brought still nearer to the glass, the influence of the flint continues for a time to increase, and the opposite condition of over-correction shows itself. But, on still further approximation of the radiant, the flint comes to operate with less effect, the excess of correction diminishes and at a point still nearer to the glass vanishes, and a second aplanatic focus appears. From this point onwards under-correction takes the place of over-correction, and increases till the object touches the surface of the glass. As every such doublet, therefore, has two aplanatic foci for all points between which it is over-corrected, while for all points beyond it is under-corrected, the optician is enabled to combine two or more doublets with perfect security against spherical error. This will be entirely avoided if the rays be received by the front glass from its shorter aplanatic focus, and transmitted through the back glass in the direction of its longer aplanatic pencil. By the approximation of the two doublets over-correction will be reduced, while their separation will produce under-cor rection ; and thus, by merely varying the distance between two such combinations, the correction of the spherical error may be either increased or diminished according to a definite rule. Slight defects in one glass may thus be remedied by simply altering its position in relation to the other, an alteration which may be made with very little disturbance of the colour-correction. This important principle was developed and illustrated by Mr Lister in a memoir read to the Royal Society on January 21, 1830, On some Properties in Achromatic Object-glasses, applicable to the Improvement of the Microscope ; and it was by work ing on the lines there laid down that the three London opticians Ross, 3 Powell, and James Smith soon pro- 2 Thus he found that, while each of Chevalier s doublet combina tions, when used singly, presented a "bur" or "coma" outwards, this coma, instead of being exaggerated by the combination of two of these doublets, was much diminished. On the other hand, while two of Tulley s triplets, each of which performed admirably by itself, were used together, the images of all objects not in the centre presented a strong bur inwards with an under-correction of colour. 3 In 1837 Mr Lister gave Mr Ross a projection for an objective of | inch focus, in which a triple front was combined with two doublets. The great superiority of this lens, admirably executed by Mr Ross, caused him to adopt its plan as the standard one for high powers; and it is still
in general use, the back lens also being sometimes made as a triplet