In order to use this microscope, the slider containing the object is thrust between the two flat brass plates EF, care being taken to put the face of the slider where the brass rings arc farthest from the eye. The magni fying glass intended for use must then be screwed on at G ; and while the eye looks through it against the sky, or the light employed, the long screw CC is turned till the objects in the slider are brought into the ante rior focus of the lens, when a distinct view of them will be obtained. By moving the sliders between the plates at E, different parts of the same object may be brought into view, or different objects in the slider ; but at every motion of the slider, vision should be made distinct by a new adjustment wiTh the screw.
This instrument is sometimes placed upon a stand, with a reflecting mirror, as shown in Fig. 15. In tit tler to see opaque objects with it in this case, an arm QR, Fig. 17. is screwed into the body of the microscope at G. The proper magnifier is next screwed into the hole at H, and putting the concave speculum S on the outside of the ring R, the object is held upon the forceps, or point of T, whose wire slips into a small hole, shown at u, in the body of the microscope. The arm R is Own turned till the magnifier is brought over the ob ject, and distinct vision is obtained by tinning the screw C as formerly.
VII. Ellis's Aquatic Microscope.
This microscope differs from the one shown in Fig. 15 so little, that it is unnecessary to engrave it. In stead of the stage DL moving, the arm AB containing the magnifiers slides up and down. The magnifiers arc each set in the centre of a concave silver speculum This microscope was employed by Ellis in his observa• Lions on and the Zoophytes. See Ellis's Es. say un the ? Natural History of Curallines, and his Na tural History of many curious and uncommon Zoophytes, VIII. Barrelled Microscopes.Viii. Barrelled Microscopes.
Single microscopes for opaque objects are sometime conveniently fitted up with a barrel or cylinder, on tht circumference of which are placed the objects to be ex. amined. By turning the barrel round its axis, the dif terent objects are brought under the magnifiers, and b3 a lateral motion of the barrel, other objects may b( brought into view.
IX. On the magnifying poner of Single Microscopes.
The magnifying power of single microscopes in creases with the smallness of their focal length, and mal be easily found, by dividing the distance at which tht eye sees objects distinctly by the focal length of tht lens or globule. This distance varies in different persons and has generally been assumed at 7 or 8 inches. It obvious, however, that very minute microscopic ohjects when examined with an eye of the ordinary focal length are always viewed at a less distance than 7 or 8 inches in order to obtain the best possible view of them ; and therefore, it is this distance that should be divided b: the focal length, in order to obtain the magnifying pow er of the instrument, or the real measure of the hell which we derive from it. This distance will be fount
not to exceed five inches ; and upon this supposition wt have computed the following Table, shewing the mag nifying power of 'single microscopes from ffith of al inch to I inch.
The first column of the Table shows the focal length of the lens, or globule, in 100c1ths of an inch. The se cond column shows the number of times that it is mag nified in diameter, or in one dimension. The third co lumn shows the number of times that the surface is magnified ; and the fourth, the number of times that the cube of an object is magnified.
As microscopic objects, however, are never mathe matical lines, and as their solidity, or their magnitude in Once dimensions, cannot be rendered visible by a mi croscope, we consider the second column as containing the real magnifying power of the microscope, although opticians have hitherto adopted the numbers in the first column, from an erroneous analogy with the telescope.
On Compound Microscopes WE have no means of ascertaining, from the descrip tions of Jansen's microscope, the particular combination of lenses which he employed. It seems more than pro bable, that the microscopes used by him and Galileo consisted of a convex object lens, and a concave eye glass, and were nothing more than a short telescope con verted into a microscope by lengthening its tube.
In 1646, Fontana tried two convex lenses ; and in all the microscopes which were subsequently used, three or more convex lenses were employed, as in the finest instruments which are now in use. Dr. Hooke, in the Preface to his Micographia, gives an account of the mi croscope which he employed. It was about seven inches long, and three inches in diameter, and consisted of four drawing tubes, by which it could be lengthened or shortened at pleasure. It had three glasses, viz. a small object-glass, a middle glass, and an eye-glass. \Vhen he wished to have a large field, or to see a great part of the object an once, he used all the three lenses—the middle tense converging upon the eye-glass pencils, which, by their divergency, could not have fallen upon it. But when he wished to examine any individual part with the greatest distinctness, he removed the middle glass, and used only the other two glasses.