One of the best modes of mounting a compound microscope is shown in the annexed view (fig. 22), which, though too small to exhibit all the details, will serve to explain the chief features of the arrangement.
A A are two uprights surmounting the tripod base, nun. At c is the axis upon which the instrument turns, and by which any inclina tion, from vertical to horizontal, may be given to it, in which position it may be clamped by the small handle u when the use of over-balanc ing apparatus on the secondary stage renders this necessary. Motion is given to the bar is by the milled head I' : a corresponding milled head on tho other aide of the instrument may also be used for the s.atne pur pose. At. the end of the arm 0 is fixed the compound tuba m a t, which receives at its upper extremity the eye-piece .1 : at. its lower extremity, 1, the objective is to be screwed on. K Lac, is the mechanical stage upon which the objects under microscopic examination are placed, the milled head marked x giving it a vertical motion, and that marked a a lateral one. This stage has also a rotary motion, concentric with the axis of the compound tube a a a, which enables the observer to examine au object in any position of rotation, keeping the object in the field of view. Fitted to the secondary stage, N, is represented a system of diaphragms employed to regulate the diameter of the pencil of light reflected from either the plane or concave surface of the mirror o. The diaphragms can be removed, and a variety of appara tus substituted. The regulation of the distance between the illumi nator and the object under examination is effected by means of the milled head P. Q is one of two milled heads which give motion in rectangular directions to the secondary stage, for the purpose of caus ing its axis to coincide with that of the compound tube. The milled head marked a communicates a rotary motion to the tube of the secondary stage. In using the higher powers, the instrument is only approximately adjusted by the milled head F, the rack with which it communicates being called the coarse adjustment : the I ne adjust ment is effected by means of the graduated milled head s, which is con nected with a screw and lever acting on the optical arrangement of an inch for each revolution of the milled head s. The milled head m is
used to clamp the arm to the bar of the instrument. This description though brief, will be found sufficiently lucid to convey to the mind of the reader correct notions of the mechanical arrangements of the com pound microscope.
Most of the directions for viewing and illuminating objects, given in reference to the simple microscope, are applicable to the compound. An argand lamp placed in the focus of a large detached lens so as to throw parallel rays upon the mirror, is the best artificial light; and for opaque objects the light so thrown up may be reflected by metallic specula (called, from their inventor, Lieberkhuns) attached to the object-glasses.
On the proposal of Sir David Brewster and M. Dujardin, the Wollaston condenser was achromatised with very excellent effect by constructing it with three pair of achromatic lenses, instead of the single lens before described. Subsequent improvements have been effected by Mr. Gillett, and a modification of the form adopted by that gentleman has been constructed by Messrs. Powell and Lealand Other modes of illumination have also been adopted, such as the prisms of Amici, Nachet, and Powell, Nobert's circular condenser, Ross's spotted lens for dark ground illumination, &c.
The erector commonly applied to the compound microscope con sists of a pair of lenses acting like the erecting eye-piece of the telescope. But this, though it is convenient for the purpose of dissection, very much impairs the optical performance of the instru ment.
For drawing the images presented by the microscope, the best apparatus consists of a mirror at (Fig. 23), com posed of a thin piece of rather dark-coloured glass, cemented on to a piece of plate-glass inclined at an angle of 45' in front of the eye-glass E. The light escaping from the eye-glass is assisted in its reflection upwards to the eye by the dark glass, which effects the further useful purpose of ren dering the paper less brilliant, and thus enabling the eye better to see the reflected image. The lens a, below the reflector, is to cause the light from the paper and pencil to diverge from the L same distance as that received from the eye ; in other words, to cause it to reach the SS.