ment may be firmly steadied in positions more or less inclined, or may be fixed upright. It may be also fixed in the horizontal position required for drawing with the Camera Lucida; for the pin at the bottom of the stem then enters the hole at the top of the stud K, and the stay G falls flat down, resting on the top of the stout pin L. The advantages of this construction are that it is strong, firm, and yet light; that the instrument rests securely at the particular inclination desired, which is often not the case on the ordinary construction when the joint has worked loose; and that in every position there is the needful preponderance of balance. The Stage D is circular, and upon it fits a circular plate T, which rotates in the optic axis of the Microscope. On the plate T there slides the Object-holder u, which is so attached to it by a wire spring that bears against its under surface, as to be easily moved by either or both hands; and as access can be readily gained to this spring by detaching the plate T from the stage, it may either be removed altogether so as to leave the stage free, or may be adjusted to any degree of stiffness desired by the observer. The object-holder has a ledge v for the support of the slide; and it is also provided with a small spring w, attached to it

FIG..49.

Collins's Harley Binocular.

by a milled-head, by turning which the spring may be brought to bear with any required pressure against the edge of the slide laid upon the object-holder, so as to prevent it from shifting its place when rotation is given to the stage, or when, the instrument being placed in the horizontal position, the stage becomes vertical. The central tube of the Stage is furnished with a rotating Diaphragm-plate, and is adapted to receive various other fittings; and a Side-Condenser on a separate stand is also supplied.'

67. Collins's Harley Binocular. This instrument, as represented in Fig. 49, is substantially framed and well hung on the Ross model; but is now made also on the Jackson model at the same price. The caps of the Eye-pieces are provided with shades, which cut off the outside lights from each eye; these can be adapted to any instrument, and the Author can speak strongly of their value from his own ex

1 The price of this instrument, with two pairs of Eye-pieces, three Objectives (a 2-inch of 10°, a 1-inch of 22°, and a 1-4th of 75°), and Side-Condenser on stand, in Case, is £16 10s.

perience. The Wenham prism at the common base of the bodies is fitted into an oblong box, which slides through the arm that carries them; this contains, in addition, a Nicol analyzing prism, and is also pierced with a vacant aperture; so that, by merely sliding this box transversely until its aperture comes into the axis, the instrument may be used as an ordinary Monocular; or, if the analyzing prism be made to take the place of the Wenham, whilst the polarizing prism beneath the stage is brought into position by rotating the Diaphragm-plate in which it is fixed, it is at once converted into a Polarizing Microscope-with the disadvantage, however, of not being then Binocular. It has also a 'nose-piece' carrying two Objectives, by a sliding movement of which one power may be substituted for the other.'

con

68. Swift's Challenge Microscope. The instrument structed under this designation by Messrs. Swift, is one of which it may be fairly said that it is surpassed by no other of its price in the excellence of its workmanship, and its suitability to the general wants of the Microscopist. The support on which it is hung is extremely firm and substantial without being heavy; and when the limb is brought to the horizontal position, resting on the cross plate between the two uprights, the instument is still well balanced. The rack and pinion movement is made with oblique teeth; a construction which favors smoothness and sensitiveness in the adjustment, so that a 1-4th inch objective may be focussed by it alone. The fine adjustment is made by the milled-head at the lower end of the body.-It is a peculiarity in this instrument, which especially fits it for those who work much with Polarized light, that the analyzing prism is fitted

into the body above the Wenham prism, in such a manner that, when its fitting is drawn out (without being removed), it is completely out of the way of the light-rays; whilst, when the use of the Polariscope is required, the prism can be at once pushed into the body, working in conjunction with the Wenham prism. This mode of mounting the analyzer is found to interfere much less with the definition of the objective, than the insertion of it between the objective and the Wenham

The price of this instrument, with Mechanical rotating Stage, two pairs of Eye-pieces, two Objectives (either a 2-inch of 12°, or a 1-inch of 18°, with a 1-4th of 95°), Side-Condenser on Stand, and Polarizing apparatus in Cabinet, is £19. Accessories of various kinds can be readily fitted to it.-A 'first-class' Binocular is also constructed by the same Maker on the Jackson model.

prism. The stage rotates in the optic axis; and may either bear (as in the figure) a sliding object-carrier, or may be furnished with mechanical actions. The mirror is attached to the stem by a crank-arm, allowing it to be so placed as to reflect light of considerable obliquity. Beneath the Stage is a broad horizontal dovetail groove, into which is very exactly fitted a firm (sprung) slide that carries a Sub-stage for illuminating apparatus, fitted with a vertical rack movement, and with horizontal centering screws; this arrangement (devised by Mr. Swift) enables the sub

stage to be placed in position or removed, without disturbing either the stage or the mirror. The extremely ingenious Universal Sub-stagecombining Achromatic Condenser, Black-ground Illuminator, and Polarizer with varied adaptations-devised by Mr. Swift for this Microscope, but capable of being applied to any other, will be described hereafter (8 112). The Author, having had his instrument (thus fitted) in constant

use for several years past, feels justified in unreservedly expressing his high appreciation of it.'

69. Browning's Smaller Stephenson Binocular.-This instrument, represented in Fig. 51, is of more substantial build than the Students' Binocular of Messrs. Baker (§ 64); and is further distinguished by its. special adaptation for use with Polarized light. In place of the reflecting prism at the junction of the inclined bodies, a plane piece of dark glass, silvered on one face, is hung on a horizontal axis at the polarizing angle; its silvered face being turned in front when it is used for ordinary purposes, so as to reflect into the two inclined bodies, the light-rays which proceed to it from the pair of dividing prisms; whilst, when it is to act as an analyzer, it is turned on its axis by means of a milled-head so as to bring the dark-glass surface to the front. Further, by fixing into the arm the tube which carries the objective, with its fine adjustment, and by making that which contains the dividing prisms and mirror, and which also carries the double body, slide over it, the latter can either be turned half round, so as to point the eye-pieces in the reverse direction (for the exhibition of the object to an observer sitting at the opposite side of a small table) without any disturbance of the adjustments; or it can be lifted off altogether, and replaced by an ordinary Monocular body.'

FIRST-CLASS MICROSCOPES.

70. We now pass to an entirely different class of Instruments-those of which the aim is, not simplicity, but perfection; not the production of the best effect compatible with limited means, but the attainment of everything that the Microscope can accomplish, without regard to cost or complexity. To such, of course, the Stereoscopic Binocular is an indispensable addition; and it is not less essential that the Stage should have a rotatory movement in the Optic axis of the instrument;-not only for the due examination of opaque objects, as already mentioned (§ 66), but also because this movement is requisite for the effective examination of very delicate transparent objects by Oblique light, allowing the effect. of light and shadow to be seen in every direction; and, in addition, because in the examination of objects under Polarized light, a class of appearances is produced by the rotation of the object between the prisms, which is not developed by the rotation of either of the prisms themselves.

71. Ross's First-class Microscope. -As what is known as the Ross model is still made, being preferred by some purchasers, we shall commence with a notice of the original form of the instrument which has gained so high a celebrity.-The general plan of this Microscope, as shown in Fig. 52, is carried out with the greatest attention to solidity of construction, in those parts especially which are most liable to tremor,

The price of this instrument in the simple form here figured, with one pair of Eye-pieces and best 1-inch and 1-4th inch (80°) Objectives, and Condensing lens on separate stand, in Case, is £14. A mechanical stage costs £2 10s. additional, and the sub-stage (without fittings) £2 2s.-A very ingenious 'swinging substage' has been lately devised by Mr. Swift (“Journ. of Roy. Microsc. Soc.," vol. iii., 1880, p. 867) for obtaining illumination of any degree of obliquity, even by two pencils at once. The Condenser is made to slide on an arc-piece (as in Mr. Grubb's arrangement, § 72), which is prolonged above the Stage for opaque illumination; and with this may be combined a second arc-piece at right angles to the first, carrying a second Condenser, which is found serviceable in the resolution of difficult Diatom-tests.

? The price of this instrument, with one pair of Eye-pieces and Objectives of 1 inch (16) and 1-4th inch (75°), is £20. Any Accessories can readily be added to it.

as also to the due balancing of the weight of its different parts upon the horizontal axis. Any inclination may be given to it; and it may be fixed in any position by a clamping screw, turned by a short lever on the righthand upright. The 'fine' adjustment is effected by the milled-head on the transverse arm just behind the base of the body;' this acts upon the nose' or tube projecting below the arm, wherein the objectives are screwed. The other milled-head, seen at the summit of the stem, serves

FIG. 52.

to secure the transverse arm to this, and may be tightened or slackened at pleasure, so as to regulate the traversing movement of the arm; this movement is only allowed to take place in one direction, namely, towards the right ride, being checked in the opposite by a 'stop,' which secures the coincidence of the axis of the principal 'body' with the centre of the stage, and with the axis of the illuminating apparatus beneath it. The object-platform, to which rectangular traversing motions are given by the two milledheads at the right of the stage, is also made to rotate in the optic axis by a milledhead placed underneath the stage on the left-hand side; this turns a pinion which works against a circular rack, whereby the whole apparatus above is carried round about two-thirds of a revolution, without in the least disturbing the place of the object, or removing it from the field of the Microscope. The graduation of the circular rack, moreover, enables it to be used as a Goniometer (§ 92). Below the stage, and in front of the stem that carries the mirror, is a dovetail sliding-bar, which is moved up and down by the milled-head shown at its side; this sliding-bar carries what is termed by Mr. Ross the 'Secondary stage' (shown separately at B), which consists of a tube for the reception of the Achromatic Condenser, Polarizing prisms, and other fittings. To this secondary stage a traversing movement of limited extent is given by means of two screws, one on the front and the other on the left-hand side of the frame which carries it, in order that its axis may be brought into perfect coincidence with the axis of the body; and a rotatory movement also is given to it by the turning of a milled-head, which is occasionally useful, and the exact amount of which is measured by a graduated circle.-The special advantages of this instrument consist in