the object; and it is advantageous that this should be made to revolve. A very convenient mode of effecting this, is to mount the selenite plate in a revolving collar, which fits into the upper end (a) of the tube (Fig. 41, B) that receives the polarizing prism. In order to obtain the greatest variety of coloration with different objects, films of selenite of different thickness should be employed; and this may be accomplished by substituting one for another in the revolving collar. A still greater variety may be obtained by mounting three films, which separately give three different colors, in a frame resembling that in which hand-magnifiers are usually mounted, so that they may be used singly or in double or triple combinations; as many as thirteen different tints may thus be obtained; but the advantage of revolution is sacrificed. When the construction of the microscope does not readily admit of the connection of the selenite plate with the polarizing prism, it is convenient to make use of a plate of brass (Fig. 43) somewhat larger than the glass

FIG. 43.

O

Selenite Object-Carrier.

slides in which objects are ordinarily mounted, with a ledge near one edge for the slide to rest against, and a large circular aperture into which a glass is fitted, having a film of selenite cemented to it; this "selenite stage" or object-carrier being laid upon the stage of the microscope, and the slide containing the object being placed upon it, the effect of the selenite is obtained, as in the previous arrangement; and by an ingenious modification contrived by Dr. Leeson, the ring into which the selenite plate is fitted being made movable, one plate may be substituted for another, whilst rotation may be given to the ring by means of a tangent-screw fitted into the brass plate. Such a "selenite stage" answers every purpose that can be required; but as there is no provision for using two or three plates in combination, it is necessary to have a distinct selenite plate for every modification of colors that may be desired.

A very beautiful effect may be obtained with certain kinds of semi-opaque objects, by illuminating them by means of a "spotted lens" (§ 61), with a polarizer of Herapathite placed at such a distance above it as to receive the converging hollow pencil near its termination in the object, and an analyzer of the usual description, a combination devised by Mr. Furze;' for the solidity which this mode of oblique illumination imparts to certain ob jects, is remarkably heightened by the play of colors afforded by the polarization of the light. When the polarizing apparatus is being employed with any but the lowest powers, it is very advan

"Transactions of the Microscopical Society" (2d series), vol. iii, p. 63.

tageous to use the achromatic condenser in combination with it; this combination, which cannot be made in ordinary microscopes, is provided for in that of Messrs. Smith and Beck, by the "cylindrical fitting" so often referred to, which can receive the polarizing prism at its lower end, and the achromatic condenser at its upper, whilst the selenite plate or plates may be interposed between them.1

FIG. 44.

64. Illuminators for Opaque Objects.-All objects through which sufficient light cannot be transmitted to enable them to be viewed in the modes already described, require to be illuminated by rays, which, being thrown upon the surface under examination, shall be reflected from it into the microscope; and this mode of viewing them may often be advantageously adopted in regard to semi-transparent or even transparent objects, for the sake of the diverse aspects it affords. Among the various methods devised for this purpose, the one most generally adopted consists in the use of a condensing lens, either attached to the microscope, or mounted upon a separate stand, by which the rays proceeding from a lamp or from a bright sky are made to converge upon the object. For the efficient illumination of large opaque objects, such as injected preparations, it is desirable to employ a "bull'seye" condenser (which is a plano-convex lens of short focus, two or three inches in diameter), mounted upon a separate stand, in such a manner as to allow of being placed in a great variety of positions. The mounting shown in Fig. 44 is perhaps one of the best that can be adopted: the

frame which carries the lens is borne at the bottom upon a swiveljoint, which allows it to be turned in any azimuth; whilst it may be inclined at any angle to the horizon, by the revolution of

1 For an account of the nature and properties of Polarized Light, which would be out of place in the present treatise, see the chapters on that subject in Dr. Golding Bird's "Manual of Natural Philosophy," Dr. Pereira's "Lectures on Polarized Light," New Ed., edited by Prof. Baden Powell, or any modern treatise on Optics.

the horizontal tube to which it is attached, around the other horizontal tube which projects from the stem; by the sliding of one of these tubes within the other, again, the horizontal arm may be lengthened or shortened; the lens may be secured in any position (as its weight is apt to drag it down when it is inclined, unless the tubes be made to work, the one into the other, more stiffly than is convenient) by means of a tightening collar milled at its edges; and finally the horizontal arm is attached to a spring socket, which slides up and down upon a vertical stem. The optical effect of such a lens differs according to the side of it turned towards the light, and the condition of the rays which fall upon it. The position of least spherical aberration; is when its convex side is turned towards parallel or towards the least diverging rays; consequently, when used by daylight, its plane side should be turned towards the object; and the same position should be given to it, when it is used for procuring converging rays from a lamp, the lamp being placed four or five times farther off on one side, than the object is on the other. But it may also be employed for the purpose of reducing the diverging rays of the lamp to parallelism, for use either with the parabolic illuminator (§ 61), or with the sidereflector to be presently described; and the plane side is then to be turned towards the lamp, which must be placed at such a distance from the condenser, that the rays which have passed through

FIG. 45.

the latter shall form a luminous circle equal to it in size, at whatever distance from the lens the screen may be held. Even where the large "bull'seye" condenser is provided, it is well to have a smaller condensing lens in addition; and this, which is usually a doubleconvex lens, may either be mounted on a separate base (Fig. 45), or may be attached. to some part of the microscope. (In Messrs. Smith and Beck's large microscope, Fig. 29, two sockets with bindingscrews, one for the condensing lens, the other for the side-reflector, are seen in the "limb.") This condensing lens is sufficient by itself for most ordinary purposes; and it may also be used to obtain a greater concentration of the rays already brought into convergence by the bull's-eye (§ 93).

Ordinary Condensing Lens.

65. The illumination of opaque objects may be effected by reflection, as well as by refraction; and a very advantageous means

FIG. 46.

of using the light of a lamp for this purpose, is afforded by the Side-Reflector contrived by Mr. Ross. This is a highly polished concave speculum (Fig. 46), which can be placed above and to one side of the object; and which is so mounted as to be capable of being placed in every kind of position, according to the place of the lamp, and the degree of obliquity of the illumination required. The squared stem, with which the speculum is connected by several intermediate joints, may be fitted to a socket, either in the stage or in some part of the microscope-stand, like that of the smaller condensing lens. The light reflected by the speculum upon the object, may be either that which falls on it direct from the lamp, or may come to it through the intervention of the bull's-eye, arranged so as to throw parallel rays upon the speculum (§ 64). The prisms already described as in use for the illumination of transparent objects by the reflection of light from beneath, may also be employed, by an inversion of their position, for the illumination of opaque objects from above. In Continental Microscopes, the prism is frequently attached to the lower end of the body; but this is an undesirable mode of supporting it, since the illumination is disturbed by every alteration in the distance between the body and the object. This seems to be provided against by the mounting of the prism in Mr. Grubb's microscope (§ 60), which allows it to be used at any angle either above or below the stage. A mode of illuminating opaque objects by a small concave speculum reflecting the light directly down upon it, was formerly much in use, but is now comparatively seldom employed. This concave speculum, termed a "Lieberkühn" from the celebrated Microscopist who invented it, is made to fit upon the end of the objective, having a perforation in the centre for the passage of the rays from the object to the lens; and it receives its light from the mirror beneath, the object being so mounted as only to stop out the central portion of the rays that are reflected upwards. The curvature of the speculum is so adapted to the focus of the object-glass, that, when the latter is duly adjusted, the rays reflected up to it from the mirror shall be made to converge strongly upon the part of the object that is in focus; consequently, unless (as is sometimes done) the speculum should be mounted on a tube sliding over the "nose" of

Side-Reflector.

the microscope, and capable of being adjusted to the different distances required by the several objectives, a separate speculum is required for every object-glass. The disadvantages of this mode of illumination are chiefly these:-first, that by sending the light down upon the object almost perpendicularly, there is scarcely any shadow, so that the inequalities of its surface, and any minute markings which it may present, are but faintly or not at all seen; second, that the size of the object must be so limited by that of the speculum, as to allow the rays to pass to its marginal portion; and third, that a special mode of mounting is required, to allow the light to be reflected from the mirror around the margin of the object. The first objection may be in some degree removed, by turning the mirror considerably out of the axis, so as to reflect its light obliquely upon the Lieberkühn, which will then send it down obliquely upon the object; the illumination, however, will not even then be so good as that which is afforded by the side-reflector. The mounting of opaque objects in wooden slides (Chapter V), which affords in many cases the most convenient means of preserving them, completely prevents the employment of the Lieberkühn in the examination of them; and they must either be set, for this purpose, upon disks which afford them no protection, or in glass cells with a blackened background. The cases wherein the Lieberkühn is most useful, are those in which it is desired to examine small opaque objects, such as can be held in the stage-forceps (§ 66), or laid upon a slip of glass, with lenses of half inch focus or less; since a stronger light can be thus concentrated upon them, than can be easily obtained by side-illumination. In every such case, a black background must be provided, of such a size as to fill the field, so that no light shall come to the eye direct from the mirror, and yet not large enough to create any unnecessary obstruction to the passage of the rays from the mirror to the speculum. With each Lieberkühn is commonly provided a blackened stop of appropriate size, having a well-like cavity, and mounted upon a pin which fits into a support connected with the under side of the stage; but though the "dark well" serves to throw out a few objects with peculiar force, yet, for all ordinary purposes, a spot made with black paper or black sealing-wax-varnish upon a slip of glass will answer the required purpose very effectually, the slip being simply laid upon the stage beneath the object.

SECTION 2. APPARATUS FOR THE PRESENTATION OF OBJECTS.

66. Stage-Forceps.-Every Microscope should be furnished with a pair of Stage-forceps (Fig. 47) for holding minute objects beneath the object-glass. They are mounted by means of a joint upon a pin, which fits into a hole either in the corner of the stage itself, or in the object-platform; the object is inserted by pressing the pin that projects from one of the blades, whereby it is sepa