The first and most important of these improvements was made about the year 1739, by Dr. Lieberkhun. He placed the single lens on the centre of a convex speculum, made of silver, and highly polished, and hav ing its concavity so adjusted to the focal length of the lens, that the light of a candle or of the sky could be thrown upon the side of the object next the observer. When the single lens is of a small focal length, the light is kept from falling upon the object, not only by its proximity to the lens, but by the body of the observer, so that it becomes impossible to examine microscopic objects, particularly opaque ones, with any satisfaction. Dr. Lieberkhun adapted a microscope to every object of any particular interest. At the eye end of a short brass tube, he placed the convave speculum, carrying the single lens in its centre. The object was placed in the middle of the tube, and had a contrivance for ad justing it to distinct vision, and at the object end of the tube, there was a plano-convex lens for concentrating the light which fell upon the concave speculum. Some of these microscopes are said to be deposited in the British Museum.
We shall now proceed to describe some of the best single microscopes, as fitted up for use. The most im portant of these are : 1. The common flower and insect microscope.
2. The small microscope, with apparatus for opaque objects.
3. \Vithering's botanical microscope.
4. The pocket botanical and universal microscope.
5. Lyonet's anatomical microscope.
9. \Vilson's pocket microscope.
7. Ellis's aquatic microscope.
8. Barrel microscope.
I. Description of the Common Flower and Insect croscofie.
This microscope is one of the most simple, and the most convenient, and is peculiarly fitted for being put into the hands of young persons, who are not capable of managing a more complicated apparatus. Above the handle (Plate CCCLXXVII. Fig. 12 ) is fixed the arm LAI, which carries a lens at I., which may be either used alone, when a small power is wanted, or in con junction with another lens a, which screws on the ring at L, when a higher power is needed. A horizontal grooved arm MN, also fixed to the handle, carries the pincers or forceps OP, which hold the object 0, and this object can be placed at different distances from the lens L, so as to obtain distinct vision by sliding the forceps along the groove in MN, and fixing it at the proper position by means of the button or nut B.
H. Description of the Microscope for Opaque Objects.
This microscope, which is a little more complicated than the preceding, is represented in Plate CCCLX X VII.
Fig. 13. The ivory handle P is fixed in the arm A, through which passes a screw B, having its other end fastened to the moveable arm C. By turning the nut D upon the head of the screw B, the arms A and C may be either separated or brought together, being kept asunder by a steel spring E. The piece GH, consisting of a pointed steel wire G, and of a pair of pliers H, passes through a spring socket moving on a rivet. The object may be either fixed on the point G, or taken up by the pliers, and may be turned round in any direction, by the joint at F, or by sliding the piece GH through the spring socket. A ring of brass I with a female screw is fixed on an upright piece of brass turning on a pivot, in order that it may be set at a proper distance when the smallest magnifiers are employed. A concave speculum K, of polished silver, has a lens placed in its centre, according to the method of Dr. Lieberkhun, already described ; and there are generally four of these of different concavities, with four glasses of different magnifying powers. A round object-plate, as shown at NI, has one side white and the other black, all dark ob jects being placed upon the white side, and all light ob jects upon the black side, in order to render them more visible by contrast. A steel spring N, moving round the centre, can be turned down on each side, for the pur pose of pressing the object to the black or white ground. A hollow pipe proceeds from the object-plate, in order to screw it on the point of the needle G. A small box 0 of brass, with a glass on each side, for the purpose of enclosing any living object for examination, has likewise a pipe for the purpose of screwing it upon the end of the needle PG. A pair of pliers for taking up any ob ject is shown at Q, and a soft hair brush at R.
In using this microscope, the speculum KL, with its accompanying magnifier, must be screwed into the ring I. The object is then to be placed according to its na ture on the needle G, in the pliers H, or on the black or white side of the object-plate NI, or between the glass plates of the box 0 ; and having taken the instrument by the handle P, the eye looks through the magnify ing lens screwed on at I, and by means of the nut D, and the motion of the needle, the object may be turn ed about, raised or depressed, or made to approach to or recede from the lens, till it is placed in the true an terior focus of the lens, where the light will be reflect ed upon it by the concave speculum.
III. Withering's Botanical Microscope.