forceps, and with metal and glass stage-plates; a dissecting-trough, lined with cork, also fits into the opening of the stage. The Simple Microscope, as used for dissecting and mounting, is shown in the lower figure; it has two powers used singly or in combination, which are carried by the smaller arm of the stand. The Compound body, as shown in the upper figure, screws into the larger arm of the stand, and has a divided objective, giving a range of three powers; the nose is made with the standard screw, so as to fit any first-class objectives. A telescope sliding arm, fitting into a socket on either side of the stage, can also be used to carry the simplemicroscope powers, as well as a larger low-power lens, that serves also as a

hand-magnifier; and the arm can be readily fixed in any desired position for examining objects away from the instrument. A watch-glass holder used upon the glass stage-plate, gives the means of sliding steadily upon the stage in any direction objects that are under examination in a watchglass. A turn-table for mounting purposes is carried upon a long spindle that works through the corner of the stage (as shown in the lower figure), the arm of the stand serving as a support for the hand whilst using the turn-table; the top is made of the size of an ordinary glass slide, and the slide is held upon it by an india-rubber band. A hot plate fits into the opening of the stage, and is heated by a spirit-lamp placed in the position of the mirror, which is then turned to one side; and the larger arm

serves also as a watch-glass holder for preparing crystals by evaporation over the spirit-lamp. A selection of materials required in preparing and mounting objects is supplied in a rack of bottles sliding in the case; and a set of instruments-dissecting-needles, knife, forceps, dipping tubes, brushes, etc.-with a supply of cover-glasses, cells, etc., are carried in the three drawers; all the different contents of the case being readily accessible when it is set open, as shown in the lower part of the figure.'

COMPOUND MICROSCOPES.

49. Of the various forms of Compound Microscope, the greater number may be grouped with tolerable definiteness into three principal Classes: the First consisting of those high-class instruments in which the greatest possible perfection and completeness are aimed at, without regard to cost; the Second including those which are adapted to all the ordinary requirements of the observer, and which can be fitted with the most important Accessories; whilst to the Third belong the Students' and Educational Microscopes, in which simplicity and cheapness are made the primary considerations. Besides these, there is a class of Microscopes devised for special purposes, but not suited for ordinary use.-In all, save the last, the same basis of support is adopted; namely, a tripod 'foot,' carrying a pair of uprights, between which the Microscope itself is swung in such a manner, that the weight of its different parts may be as nearly as possible balanced above and below the centres of suspension in all the ordinary positions of the instrument. This double support was first introduced by Mr. George Jackson, who substituted two pillars (a form which Messrs. Beck retain in their Large Microscope, Plate VII.,. and is now adopted by Messrs. Ross, Plate v.) for the single pillar, connected with the Microscope itself by a 'cradle joint,' which was previously in use, and which is still employed in many Continental models (Fig. 45). But in place of pillars screwed into the tripod base, the uprights are now usually cast in one piece with the base, both for greater solidity and for facility of construction (Fig. 39); while in most of the more recent models an open framework is adopted (more or less resembling that first devised by Mr. Swift, Fig. 50), which combines great steadiness with lightness. Messrs. Powell and Lealand, it will be observed, adopt a tripod support of a different kind (Fig. 48 and Plate VI.); still, however, carrying out the same fundamental principle of swinging the Microscope itself between two centres. An entirely new and very effective mode of swinging the body has lately been introduced by Mr. George Wale of New York (Fig. 44).-Two different modes of giving support and motion to the body' will be found to prevail. In the first, which may be called the Ross model (as having been originally adopted by Mr. Andrew Ross), the body' is attached at its base only to a transverse arm,' which, being pivoted to the top of the stem,' is raised or lowered with it by the rackand-pinion action that works in the pillar to which the stage is fixed (Fig. 52). The fundamental objection to this method is, that unless the

1 The whole of this apparatus is supplied complete at the moderate cost of £1. or, without the Compound body and inclined movement of the stand, at £2 10s. 2 It is true that the most important of these accessories may be applied to some of the smaller and lighter kind of Microscopes; but when it is desired to render the instrument complete by the addition of them, it is far preferable to adopt one of those larger and more substantial models, which have been devised with express reference to their most advantageous and most convenient employment.

6

transverse arm and the body are constructed with great solidity, the absence of support along the length of the latter leaves its ocular end subject to vibration, which becomes unpleasantly apparent when high powers are used, giving a dancing motion to the objects. With the view of preventing this vibration, the top of the body is sometimes connected. with the back of the transverse arm by a pair of oblique 'stays' (Fig. 48); but the usual plan is to obtain the requisite firmness by the thickness and weight of the several parts. In the other, which may be termed the Jackson model, and which was first adopted by Mr. James Smith (the predecessor of Messrs. Beck), the body is supported along a great part of its length on a solid limb' whereby its vibration is reduced to a minimum; and the rack, which is acted on by a pinion working in that limb, is attached to the body itself; a construction that gives a great smoothness and easiness of working (Plate VII.).—Having made use of instruments constructed by the best makers on both models, the Author has no hesitatation in expressing his preference for the second, which is now employed by most English makers (having been adopted by Messrs. Ross themselves in their more recent instruments), and by nearly all American. He regards it as certain that greater freedom from vibration. can be obtained in lightly-framed Microscopes constructed on the Jackson model, than in any but the most solid and cumbrous of the old Ross pattern; and feels assured that the principle of supporting the body' along a great part of its length (which may be applied in a variety of modes) will in time supersede that of fixing it by its base alone, which is oviously the mode least adapted to prevent vibration at its ocular end.

In describing the Instruments which he has selected as typical of the several groups above enumerated, the Author wishes not to be understood as giving any special preference to these, above what may be the equally good instruments of other Makers. The number of those who now construct really excellent Microscopes has of late years increased greatly; but their models are for the most part copied more or less closely from those previously adopted for their high-class Microscopes by the three principal Firms which long had exclusive possession of the field. Where any individual Maker has introduced a real novelty, either in plan of construction, or in simplification leading to reduction of price, the Author has thought this worthy of special notice whilst the limits within. which he is restricted oblige him to content himself with a bare mention of other Makers whose productions are favorably known to him. It will be found most advantageous to commence with the Educational and Students' Microscopes, as the most simple in construction; and to proceed from these through the Second to the First-Class Microscopes, reserving to the last the group of instruments adapted for Special purposes.

THIRD-CLASS MICROSCOPES.

50. Very important contributions to our knowledge of Nature have unquestionably been made by the assistance of instruments not surpassing the least perfect of those now to be described. And there is this advantage in commencing Microscope-work with a simple and low-priced instrument—that the risk of injury to a more costly Microscope, which necessarily arises from want of experience in its use, is avoided; whilst the inferior instrument will still be found serviceable for many purposes,

after a better one has been acquired. Microscopes, of whatever Class, should be provided with the Society's screw' now used not only by British and American, but also by several Continental Makers; so that any of their. Objectives may be fitted to them. (See Note, p. 58.)

Educational Microscopes.

51. Field's Educational Microscope. This instrument is known as the Society of Arts Microscope,' in consequence of its having gained the medal awarded by that Society, in 1855 (at the suggestion of the Author) for the best three-guinea Compound Microscope that was then produced. It has two Eye-pieces, and two achromatic Objectives, Condenser, Livebox, etc., and retains its place amongst useful instruments of low price. It is within the knowledge of the Author, that the production of this instrument has greatly promoted the spread of Microscopy among many to whom the pursuit has proved most valuable as a refreshing and elevating occupation for hours that might have been otherwise either spent in idleness or turned to much worse account.

Fro.. 88,

52. Crouch's Educational Microscope.-This is a very simple and at the same time serviceable, instrument (Fig. 38); well suited for the display of Botanical objects, small Insects or parts of larger ones, Zoophytes and Polyzoa that may be picked up on almost any sea-shore, or the Circulation in a Frog's foot. In order to minimize its cost, the ordinary modes of focal adjustment are dispensed with; the 'coarse' adjustment being made by sliding the body through the tube which grasps it, and which is lined with velvet to secure a smooth and equable 'slip;' and the fine' by slightly drawing-out the Eye-pieces. This method answers very well for the low powers for which this instrument is intended; and it has the advantage of not allowing the adjustment which a Teacher has made, to be readily disturbed by the Pupils to whom an object is being exhibited. It is provided with a side-condenser for illuminating opaque objects; and with a diaphragm-plate fitted into a tube which is screwed into the aperture of the stage, and which is adapted also to receive a polarizing prism and spot-lens."

53. Parkes's Educational Microscope.-Such as desire a large and more substantial instrument, which may be advantageously used for higher powers, and made to serve a greater variety of purposes, will find the Microscope represented in Fig.

Crouch's Educational Microscope.

39 very suitable to such requirements. It is solidly built, without being unduly weighty, carries a body of full diameter (which can be lengthened by a draw-tube to ten inches), and stands well upon its base. The 'coarse' adjustment is made (as in the preceding case) by sliding the body within the tube that grasps it, the lining of which with cloth makes it work very easily (Fig. 39); but a rack and pinion movement may be added at a

small additional cost. The 'fine' adjustment is made by a screw (turned by the milled-head at the top of the vertical pillar), which acts on the carriage of the body; and as this carriage slides between dove-tailed grooves, the adjustment is made with entire freedom from twist.' The Microscope is furnished with two eye-pieces, of which the lower is preferable for objects requiring good definition; whilst the higher gives a flat field of eight inches