vented an admirable Student's Microscope, which commends itself to all who desire efficiency, cheapness, and portability. The accompanying cut explains its appearance and mode of action. It is mounted on a tripod base, with uprights of japanned cast iron. It has a quick and a slow movement, with draw-tube and a stage 3 by 4 inches, movable by a lever. This movement the Messrs. G. have nearly perfected, and at the suggestion of Dr. Goddard, of this city, an eminent microscopist, they have so arranged it that the stage follows the hand instead of taking the opposite direction, as is usually the case in other instruments having the lever stage. (Fig. 428.)

Messrs. Grunow have also improved their stand in the mode of adaptation of accessory apparatus, especially of that which is attached below the stage; this is held in place by a bayonet.

catch, and when once centred it may be detached and reattached with the greatest facility, without requiring a readjustment.

Fig. 429 represents a smaller and less expensive form of student's microscope, made by the Messrs. Grunow, and possessing the same objectives as that represented in Fig. 428, but with a non

movable stage. Messrs. Grunow also manufacture a first class microscope, with brass mountings and all the accessories. (Fig. 430.) Their instruments have given great satisfaction wherever they have been used.

THE INVERTED MICROSCOPE OF DR. J. LAWRENCE SMITH.

This instrument was invented and brought to the notice of the Société de Biologie of Paris, in 1850, by Dr. Smith, for whom

the first one was constructed by Nachet, of Paris. Its design is to enable the chemist to carry on observations under the microscope without the risk of injuring his objectives, or at least having his view obscured, by the fumes arising from liquids experimented upon. This has always been a serious difficulty in

micro-chemical research, and, together with the difficulty of manipulating in the limited space between the object-glass and the stage, has hitherto prevented this branch of scientific inquiry from being fully illustrated. "The only way," says Dr. Smith, "by which these difficulties can be surmounted, is by putting the object-glass beneath the stage and the object above it, with an optical arrangement of such a nature as to permit observation. It was with this view that M. Chevalier made a chemical

support, to go with his general instrument, and so constructed it, that it could be inverted so as to have the stage above, and the objective below the object; but every one who has used it knows how awkward it is for manipulation, although exceedingly ingenious. Dr. Smith, impressed with these difficulties, was led to the construction of the instrument, which, in this country, bears his name.

It was important for the arrangement in question, so to have the relative position of the stage and eye-piece, that the eye, while on a level with the latter, could readily see the former, and guide the required manipulation.

E

FIG. 431.

D

The most important part of the instrument is a four-sided prism, Fig. 430, with the angles ab, be, ed, and da, respectively 55°, 107°, 52°, 145°, the angles being of such dimensions that a ray of light passing into the prism in the direction Ea, and perpendicular to the upper surface of the prism, after undergoing total reflection from the inner surfaces (on both of which the light strikes at an angle much less than forty-five degrees), will pass out perpendicular to the surface connected with the body of the instrument.

It will be readily seen how a ray of light, entering the

a

A

object-glass, descends into the prism, and passes out of it upwards, through the eye-glass, the tube of which is inclined to the perpendicular 35°. The other parts of the instrument will be understood by looking at the figure. (Fig. 431.) The illumination of the object is effected by a prism, instead of a mirror.

In examining an object with this microscope, the object is arranged in the ordinary way; when liquid, it is placed in a watch-glass, or such glass cells as are convenient to use. In employing reagents, they can be added and watched immediately, for it is readily seen how the eye guides the manipulation on the stage and looks into the instrument almost at one and the same time.

In observing with high powers, as the object-glass is beneath the glass supporting the object, and as the glass is usually of a certain thickness, the method of observation must be changed. For all powers resorted to in chemical examination this difficulty never occurs, and in using high powers it is easily obviated. When the object is already mounted and dry, the thin glass can be readily turned downwards; but where it is moist, as for in

stance, in examining fresh Desmidia and Diatomaceo, the following plan is resorted to, namely: to use a cell, made of a thin piece of brass or glass, perforated with a hole, about a half an inch in diameter; it is best to give the hole a considerable bevel in one direction, as it facilitates the cleaning of it. Over the small end

of the hole a piece of thin glass is stuck, with balsam or other cement. When used, the object to be examined is placed within, and a cover of thin glass placed above. When brass is used to make the cell, it may be as thin as the twentieth of an inch. Dr. Smith remarks that for all observation with high powers, the Inverted Microscope is decidedly superior to the ordinary forms of mounting, for in the latter case, when an object-glass of ath or th inch focus is used, the focus is too short to admit of the use of cells; whereas, in the inverted form, as the object is looked at from beneath, the cell may be as thick as one pleases. Another thing connected with this class of observations, is that the Diatomaces and Desmidiæ can be observed to much greater advantage from beneath than from above, for reasons that will be obvious to persons accustomed to observe these classes of objects.

Another advantage possessed by this instrument calculated to extend its use for general purposes, is its great capacity for every variety of illumination, without sacrificing the ease and freedom