" Several small glass vessels must then be provided, having different depths, from one inch to three inches, and having their bottom composed of a piece of flat glass, for the purpose of admitting freely the reflected light• which is intended to illuminate the object. The fluid in which the object has been preserved, or prepar ed, is next put into the vessel ; and the object itself, placed upon a glass stage, or if necessary fixed to it, is immersed in the fluid. The glass vessel is now laid upon the arm of the microscope, which usually holds the object, and the lens is brought into contact with the the fluid in the vessel. The rays which diverge from the object emerge directly from the fluid into the ob ject-glass, and therefore suffer a less refraction than if it had been made from air ; but the focal length of the lens is very little increased, on account of the great ra dius of its anterior surface. The object may now be observed with perfect distinctness, unaffected by any agitation of the fluid ;—its parts will be seen in their finest state of preservation ;—delicate muscular fibres, and the hairs and down upon insects, will be kept sepa rate by the buoyancy of the fluid ; and if the object when alive, or in its most perfect state, had a smooth surface, its natural polish will not only be preserved but heightened by contact with the fluid. Aquatic plants and animals will thus be seen with unusual dis tinctness, and shells and unpolished minerals will have a brilliancy communicated to their surfaces, which they could never have received from the hands of the lapi dary. If the specific gravity of the substance under examination should happen to be less than that of the fluid, and it it cannot easily be fixed to the glass stage, it may be kept from rising to the top by a piece of thin parallel glass, or by a small grating of silver wire stretched across the vessel.
" The method of fitting up and using the compound microscope, which has now been described, enables us, in a very simple manner, to render the object-glass per fectly achromatic, without the assistance of any addi tional lens. The rays which proceed from the object immersed in the fluid, will form an image of it nearly at the same distance behind the lens, as if the object had been placed in air, and the rays transmitted through a plain concave lens of the fluid combined with the object glass. if we, therefore, employ a fluid whose disper sive power exceeds that of the object-glass, and accom modate the radius of the anterior surface of that lens to the difference of their dispersive powers, the image will be formed perfectly free from any of the primary co lours of the spectrum. The fluids most proper for this purpose are, 44 These oils arc arranged in the order of their disper sive powers ; and when those at the top of the list are used, the anterior surface of the object-glass will re quire a greater radius of curvature than when those at the bottom of the list are emp.oyed. Thus, in order to tender the object-glass achromatic, when it is made of crown glass, and when the fluid is oil of cassia, the ra dius of the anterior or immersed surface, should be to that of the surface next the eye as 2.5 to I. Lest these
proportions should not exactly correct the chromatic aberration, it be preferable to make the radii as 2.2 to 1, and then reduce the dispersive power of the oil of cassia by oil of olives, or any other less disper sive oil, till the correction of colour is complete. If the oil of sweet fennel seeds is used, the radius of the at), terior should be to that of the posterior surface, as o.'s to i." VII. On the Magnifying Power of Compound scopes.
When the microscope consists of two lenses, or of one speculum and one lens, the object is magni fied from two causes ; first, from the enlargement of the image produced by the object-lens or specu lum, which is always the quotient, arising from dividing the distance of the image from the ob ject•lens or speculum, by the distance of the object from the same ; and, secondly, from the effect produced by the eye-glass, which is always equal to the quotient arising from dividing the distance at which the eye sees distinctly by the focal length of the eye-glass. The product of these two quotients will therefore be the magnifying power ofthe microscope. Hence, callingf the focal length of the eye-glass, D the distance of the object from the object-glass, d the distance of the image, and A the distance at which the eye sees objects distinctly, we shall have the magnifying power or M, by the fol lowing formula, d A M If a third, or amplifying glass is added, as in the common compound microscope, for the purpose of en larging the field, the magnifying power of the instru ment is diminished, and may be found by multiplying its magnifying power without the amplifying glass, as given by the above formula, by the fractioncp be ep ing the focal length of the amplifying lens and L = — f, (3` being the distance of the ep first and second glasses, and d' the distance of the first and third glasses. Hence we obtain the following general rule. Divide the difference between the dis tance of the two first lenses, or those next the object, and the focal distance of the second or amplifying glass, by the focal distance of the second glass, and the quotient will be a first number. Square the distance between the two first lenses, and divide it by the difference be tween that distance and the focal distance of the se cond glass. From this quotient subtract the excess of the distance between the first and third glass above the focal length of the third glass, and divide the remain der by the focal distance of the third glass, or that next the eye, and a second number will be obtained. Mul tiply together the first and second numbers, and the magnifying power of the object-glass as found from the rule in p. 243. col. I, and the product will be the magnifying power of the compound microscope.