In the year 1823 a strong desire became manifest for improved forms of the instrument, in France by M. Selligue, by Frauenhofer in Munich, by Amici in Modena, by M. Chevalier in Paris, and by Dr. Goring, Mr. Pritchard, and Mr. Tully in London. The result was that in 1824 a new form of achromatic object-glass was constructed of nine-tenths of an inch focal length, composed of three lenses, and transmitting a pencil of eighteen degrees; and which, as regards accurate correction throughout the field, was for some years regarded as perfect.
Sir David Brewster was the first to suggest the great importance of introducing materials of a more highly refracting nature into the construction of lenses. He wrote: “There can be no essential improvement expected in the microscope unless from the discovery of some transparent substance which, like the diamond, combines a high refractive with a low dispersive power.” Having experienced the greatest difficulty in getting a small diamond cut into a prism in London, he did not conceive it practicable to grind, polish, and form it into a lens.
Mr. Pritchard, however, was led to make the experiment, and on the 1st of December, 1824, “he had the pleasure of first looking through a diamond microscope.” Dr. Goring also tried its performance on various objects, both as a single microscope and as an objective of a compound instrument, and satisfied himself of its superiority over other kinds of lenses. But here Mr. Pritchard’s labours did not end. He subsequently found that the diamond used had many flaws in it, which led him to abandon the idea of finishing it. Having been prevented from resuming his operations on this refractory material for a time he made a third attempt, and met with another unexpected defect; he found that some lenses, unlike the first, gave a double or triple image instead of a single one, in consequence of some of their parts being either harder or softer than others. These defects were found to be due to polarisation. Mr. Pritchard having learned how to decide whether a diamond is fit for a magnifier or not, subsequently succeeded in making two planoconvex lenses of adamant; these proved to be perfect for microscopic purposes. “One of these, of one-twentieth of an inch in focal length, is now in the possession of his Grace the Duke of Buckingham; the other, of one-thirtieth of an inch focus, is in his own hands.”
“In consequence of the high refracting power of a diamond lens over a glass lens, the former material may be at least one-third as thin as that of the latter, and if the focal length of both be equal, say, one-eightieth of an inch, the magnifying power of the diamond lens will be 2,133 diameters, whereas that of glass will be only 800.” At a date (1812) before Brewster proposed diamond lenses he demonstrated a simple method of rendering both single and compound microscopes achromatic. “Starting,” he says, “with the principle that all objects, however delicate, are best seen when immersed in fluid, he placed an object on a slip of glass, and put above a drop of oil, having a greater dispersive power than the single concave lens, which formed the object-glass of the microscope. The lens was then made to touch the fluid, so that the surface of the fluid was formed into a concave lens, and if the radius of the outward surface was such as to correct the dispersion, we should have a perfect achromatic microscope.” Here we have the immersion system foreshadowed. Shortly after these experiments of Brewster’s were in progress, Dr. Goring is said to have discovered that the structure of certain bodies could be readily seen in some microscopes and not in others. These bodies he named test objects. He then examined these tests with the achromatic combinations of the Tullys, and was led to the discovery that “the penetrating power of the microscope depends upon its angle of aperture.”
“While these practical investigations were in progress,” writes Andrew Ross, “the subject of achromatism engaged the attention of some of the most profound mathematicians in England, Sir John Herschel, and Professors Airy and Barlow. Mr. Coddington and others contributed largely to the theoretical examination of the subject; and although the results of their labours were not applicable to the microscope, they essentially promoted its improvement.”
About this period (1812) Professor Amici, of Modena, was experimentally engaged in the improvement of the achromatic object-glass, and he invented a reflecting microscope superior to those of Newton, Baker, or Smith, made as early as 1738, and long ago abandoned. In 1815 Amici made further experiments, and introduced the immersion system; while Frauenhofer, of Munich, about the same time constructed object-glasses for the microscope of a single achromatic lens, in which the two glasses, although placed in juxtaposition, were not cemented together.
Dolland, it has been said, introduced achromatic lenses; but although he constructed many achromatic telescopes, he did not apply the same principle to microscopes, and those which he sold were only modifications of the compound microscope of Cuff.
Dr. Wollaston employed a new form of combination in a microscope constructed for his own use, and by which “he was able to see distinctly the finest markings upon the scales of the Lepisma and Podura, and upon those of the gnat’s wing.” His doublet is still employed, and to which I shall refer under “Simple Microscopes.”
Fig. 3a.—Sir David Brewster’s Microscope, of the early part of the century, recently presented to the British Museum.