”We shall first speak of the single microscope, that having invented and used long before the double or compound microscope. When the lenses of the single microscope are very convex, and consequently the magnifying power great, the field of view is small ; and it is difficult to adjust with accuracy their focal distance, that it requires some practice to render the use of them familiar. It was with an instrument of this kind that Leeuwenhoek and Swammerdam, Lyonet and Ellis, examined the invisible forms of nature, and by their example stimulated others to the same pursuit.

About the year 1665, small glass globules began to be occasionally applied to the single microscope, instead of convex lenses; and by these globules an immense magnifying power was obtained. Their invention has been generally attributed to M. Hartsoeker; though it appears that we are really indebted to the celebrated Dr. Hooke for this discovery, for he described the manner of making them in the preface to his Microscophia Illustratrata, published in the year 1656.

Mr.Stephen Gray having observed some irregular particles within a glass globules, and finding that they appeared distinct and prodigiously magnified when held close to his eye, concluded, that if he placed a globule of water in which there were any particles more opaque than the water near his eye, he should see those particles distinctly and highly magnified. His method was, to take on a pin a small portion of water which he knew contained some minute animalcules enormously magnified; for those which were scarcely discernible with his glass globules, with this appeared as large ordinary-sized peas.

Dr. Hooke thus describes the method of using this water-microscope: If you desirous he says, a of obtaining a microscope with one single refraction, and consequently capable of procuring the greatest clearness and brightness any one kind of microscope is susceptible of, spread a little of the fluid you intend to examine on a glass plate; bring this under one of your globules, then move it gently upwards till the fluid touches the globule, to which it will soon adhere, and that so firmly as to bear being moved a little backwards or forwards. By looking through the globule, you will then have a perfect view of the animalcules in the drop.

The construction of the single microscope is so simple, that it is susceptible of but little improvement, and has therefore undergone few alterations; and these have been chiefly confined to the mode of mounting it, or to addition to its apparatus. The greatest improvement this instrument has received was made by Lieberkuhn, about the year 1740; it consists in placing the small lens in the centre of a highly-polished concave speculum of silver, by which means a strong light is reflected upon the upper surface of an object, which is thus examined with great ease and pleasure.

Before this contrivance, it was almost impossible to examine small opaque objects with any degree of exactness; for the dark side of the object being next the eye, and also overshadowed by the proximity of the instruments, its appearance was necessarily obscure and indistinct. Lieberkuhn adapted a separate microscope to every object: but all this labour was not bestowed on trifling objects; his were generally the most curious anatomical preparations, twelve of which, with their microscopes, are deposited in the Museum of the Royal College of Surgeons.