The history of the modern microscope, like that of nations and arts, has had its brilliant periods, in which it shone with uncommon splendour, and was cultivated with extraordinary ardour; these periods have been succeeded by intervals marked with no discovery, and in which the science seemed to fade away, or at least to lie dormant, till some favourable circumstance—the discovery of a new object, or some new improvement in the instruments of observation—awakened the attention of the curious, and reanimated the spirit of research. Thus, soon after the invention of the microscope, the field it presented to observation was cultivated by men of the first rank in science, and who enriched almost every branch of natural history by the discoveries made by means of this instrument.
The Modern Microscope.
To the celebrated Dr. Hooke belongs the honour of publishing an account of the compound instrument in 1665 in his “Micrographia.” His first claim, however, is founded on the application of a lamp adjustable on a pillar, together with a glass globe of water and a deep plano-convex condensing lens. By means of this arrangement, he says, “The light can be directed more directly on the object under examination.” In the further description given of his microscope, he explains: “It has four draw-tubes for lengthening the body, and a third lens to the optical combination.” This, it would appear, was only brought into use when he wished to see the whole object at once: “The middle-glass lens, conveying a very great company of radiating pencils (of light) which would stray away; but when I had occasion to examine the small parts of a body, I took out the middle glass and made use of one eye-glass with the object-glass.”
From Hooke’s description I gather that he also introduced the ball-and-socket movement into the construction of the body of his instrument. This has found many imitators since his day; some of them have gone so far as to claim the invention as one quite new. For small accessories, where the leverage need not be considered, the ball-and-socket has proved convenient enough; but not, however, if applied to the stand of the microscope. Hooke, in his early work, expressed dissatisfaction with the English-made lenses he had in use. He complains of the “apertures of the object-glasses, which are so small that very few rays are admitted; none will admit a sufficient number of rays to magnifie the object beyond a determinate bigness.” So we may take it that he thus early discovered the great importance of an increase in the aperture of his microscope. Other improvements of importance were made, and he was the first to describe a useful method of estimating the magnifying power of his lenses, and the difficulty of distinguishing between a prominence and a depression in the object under investigation, which he was made more fully aware of when preparing drawings for the illustration of his “Micrographia Illustrata”; this would be in 1664, if not earlier. His book created no little sensation on its first appearance, and it soon became scarce. Hooke (says Mr. Mayall) “must undoubtedly be credited with the first suggestion of immersion lenses.” Nevertheless, in his “Lectures and Collections,” published in 1676, he appears to be no longer enthusiastic over his double microscope, and once more he reverts to the simpler instrument of his earlier days. Whether this change of opinion was due to the publication of Leeuwenhoek’s observations with his simple microscopes it is impossible to say.
As early as 1673 Leeuwenhoek communicated some important discoveries made by a simple microscope of his own construction to the Royal Society; he, however, gave no particulars of the construction of the instrument. Dr. Adams, writing to his friend (Sir) Hans Sloane, says: “They appear to be spherules lodged between two plates of gold or brass, in a hole whose diameter appears to be no bigger than that of a small pin’s head.” At his death he bequeathed to the Royal Society a cabinet containing twenty-six of these microscopes; the cabinet and the microscopes long ago disappeared, but not before they were carefully examined and described by Mr. Henry Baker, F.R.S. In his report to the Royal Society, he says: “They consisted of a series of convex-lenses, ranging in power from 1·20 to 1·5, and magnifying from 160 to 40 diameters.” This must now be regarded as an eventful period in the history of the microscope, since Leeuwenhoek’s discoveries created a great sensation throughout Europe. And all further improvements in compound instruments appear to have been laid aside for some considerable period in consequence: and the pocket instrument of Wilson, together with that of his scroll standard (seen on the cover of this book), and which was one of the first simple microscopes with a mirror mounted on the base in a line with the optic axis.
The discoveries once more made, and at a much later period (1738), by Dr. Nathaniel Lieberkuhn with his simple microscopes, and by means of which he discovered the minute structure of the mucous membrane of the alimentary canal, and which alone would have immortalised his name had we not preserved in use to this day an important adjunct of every modern instrument, the Lieberkuhn reflector.
In the Museum of the Royal College of Surgeons of England, there is a small cabinet of two drawers, containing a set of twelve of his simple microscopes, each being provided with an original injection. The form of the instrument is shown in Figs. 1 and 2. a b represents a piece of brass tubing about an inch long and an inch in diameter and provided with a cap at each extremity. The one at a carries a small double-convex lens of half an inch focal length; while at b there is fixed a condensing lens three-quarters of an inch in diameter. In [Fig. 2] the instrument is seen in section, and explains itself. It is held by the handle in such a position that the rays of light, from a lamp or a white cloud, may fall on the condenser b, and concentrate on the speculum l. This again further condenses the rays on the disc c, where the object is held, and its adjustment made by the milled-head screw d, so as to bring it within the focus of the lens a.
Fig. 1.
From this digression I pass on to the evolution of the compound microscope. The earliest workable form known was that designed by Eustachio Divini, who brought it to the notice of the Royal Society in 1668. It consisted of two plano-convex lenses, combined with their convex surfaces retained in apposition. His idea was subsequently improved upon by a London optician. Not long afterwards, Philip Bonnani published an account of his improved compound microscope; and we are certainly indebted to him for two or more forms of the movable horizontal microscopes, and for the compound condenser fitted with focussing gear for illuminating transparent objects by transmitted light. I must, however, pass by the many changes made in the structure and form of the instrument by the celebrated Dr. Culpeper, Scarlet, Cuff, and many other inventors.