We now closely approach a period fertile in the improvement of the microscope, and in the discoveries made by its agency. The chief of those among the honoured names of the time we find Trembley, Ellis, Baker, Adams, Hill, Swammerdam, Lyonet, Needham, and a few others. Adams somewhat sarcastically observes “that every optician exercises his talents in improving (as he calls it) the microscope, in other words, in varying its construction and rendering it different in form from that sold by his neighbour; or at the best rendering it more complex and troublesome to manage.” There were no doubt good reasons for these and other strictures upon inventors as well as makers of microscopes, even in the Adams’ day. In the year 1787 the “Microscopical Essays” of his son were published, in which he described all the instruments in use up to that period.
Looking back, and taking a general survey of the work of nearly two centuries in the history of the microscope, it cannot be said that either in its optical or mechanical construction any great amount of progress was made. This in part may have arisen from the fact that no pressing need was felt for either delicate focussing or higher magnification. At all events, it was not until the application of achromatism to the instrument that new life was infused into its use, and a great impetus was given to its development, both optically and mechanically.
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.