not be remitted, until the varnish is dry enough to hold down the cover by itself. For mounting delicate objects, the thinner slides should be selected (§ 116); and for very difficult test-objects, it is advantageous to employ thin glass below as well as above the specimens, for the sake of diminishing the aberration which the illuminating pencil sustains in its passage to the object, and for allowing the achromatic condenser to approach the object as closely as possible. For this purpose, the simplest method is to take a slip of wood, of the ordinary size of the glass slide (3 in. by 1 in.), with a central aperture of from 3 to 5-8ths of an inch; to cover this aperture with a "square" or "round" of thin glass of sufficient size to project considerably beyond it; to lay the object upon this glass, and to protect it with a cover of rather smaller size, which should be fastened down all round by varnish, to prevent the entrance of moisture; and finally to secure both glasses to the wooden slide, by gumming down over them a piece of paper, with a perforation of the same size as that of the slide itself.

123. For dry-mounting opaque objects, the method adopted must vary with the mode in which the object is to be illuminated; since, if a side-condenser or reflector is to be employed, the whole slide may be opaque; whereas if the Lieberkühn is preferred, the object should be placed on a disk of appropriate size (§ 65), supported in such a mode as to admit the light all around it. For the former purpose, the Author has devised the following simple method, which he has found to afford peculiar conveniences. Let there be provided a cedar slide of the kind just described, a piece of card of the same dimensions, and a piece of dead-black paper, rather larger than the aperture of the slide, if a dark mounting be desired, which is preferable for most objects; this piece of paper is to be gummed to the middle of the card, and then, some stiff gum having been previously spread over one side of the slide (care being taken that there is no superfluity of it immediately around the aperture), this is to be laid down upon the card, and subjected to pressure. An extremely neat "cell" will thus be formed for the reception of the object, the depth of which will be determined by the thickness of the slide, and the diameter by the size of the perforation; and it will be found convenient to use slides of various thicknesses and having apertures of different sizes. The cell should always be deep enough for its wall to rise above the object; but, on the other hand, if it be too deep, its wall will interfere with the oblique incidence of the light upon any object that may be near its periphery. The object, if flat or small, may be attached by ordinary gum-mucilage; if, however, it be large, and the part of it to be attached have an irregular surface, it is desirable to afford a "bed" to this by gum

'It will be found a very convenient plan, to prepare a large number of such slides at once; and this may be done in a marvellously short time, if the slips of card have been previously cut to the exact size in a bookbinder's press.

thickened with starch. The complete protection thus given to the object, is the great recommendation of this method; since, when objects are simply fastened on black paper gummed on a slip of glass, their protection from its surface renders them constantly liable to accidents; as many know, to their cost, who have used that mode of mounting. But this is by no means its only convenience. It is far cheaper than mounting objects in glass cells, which is the only other mode of affording them protection, save the use of pill-boxes in the manner to be presently described. It allows the slides, not only to range in the ordinary cabinets, but also to be laid one against another, and to be packed closely in cases or secured by elastic bands; and this last plan is extremely convenient, not merely for the saving of space, but also for preserving the objects from dust. Should any more special protection be required, a thin glass cover may be laid over the top of the cell, and secured there by gummed paper; but this will, of course, occasion a slight projection, which will expose the glass cover to the risk of fracture when the slide is pressed against others; and the Author's experience leads him to conclude, that the mode of packing just described affords a security from dust, that is scarcely less effectual than a thin glass cover. Further, the card on the under surface affords a great convenience for writing on the slide the name and other particulars of the object. If the object be so large as to project above the surface, even when the thickest slides are used which it is convenient to employ, an additional protection may be afforded, by gluing a couple of strips of wood of adequate thickness along the edges of the slide;' or by gumming a second slide to the face of the first, taking care that its aperture be large enough to prevent obstruction to oblique light. Very delicate, flat objects, on the other hand, even when to be viewed by incident light, should be mounted on glass and protected by a cover, in the same manner as transparent objects; a dark background being furnished to them, either by the "dark well" (§ 65) or by the closed diaphragm (§ 55).

124. Objects which are to be viewed by the Lieberkühn should be mounted either on flat disks of card or cork, or, if protection be desired, in cups of proper depth, resembling very shallow pillboxes, which may be made with or without covers,' of any size

A very convenient kind of slide, for mounting large opaque objects, is made by Messrs. Carpenter and Westley, 24 Regent Street, by ploughing out a groove in a strip of wood, so that its section presents this form U. The object may be mounted upon the bottom of the groove; or upon a disk of card fitted into an aperture in the slide, and so held there by a brass ring, that it may be taken out and held between the stage-forceps, so as to be viewed with the Lieberkühn. These slides are commonly made five inches long, and are fitted for the reception of four objects; for the sake of uniformity, however, as well as for preventing them from overbalancing on the stage, it might be convenient that they should be made three inches long, and that each should only receive one or (at most) two objects.

2 Round boxes with glass covers are now coming into very extensive use for the preservation of Natural History specimens of various kinds, in such a manner that the con

desired, and lined with black paper, by any pill-box maker. The disks or pill-box cells may be attached to glass slides by gum, so as to range in an ordinary object-cabinet; and this mode will be generally adopted by such as lay stress upon uniformity, and prefer the easiest methods of exhibiting their objects. As there are many opaque objects, however, which can only be well judged of when different sides are presented to the Microscope, there is a great advantage in mounting these in such a manner, as to admit of their being turned at various angles; and this may be done by attaching the disk with sealing-wax or some other cement, to a pin, which may be either held between the blades of the stage-forceps, or passed into the cork box at its other extremity (§ 66). If the Microscopist should be pursuing the study of any class of objects which renders it desirable to mount a large number in this mode, the most convenient plan is to glue two pieces of cardboard to the two sides of a piece of rather thick chamois leather; one of the surfaces of this sandwich-like board should be covered with dead-black paper; and disks of any desired diameter may then be cut out with a punch, and mounted upon a pin by simply passing it through the stratum of leather. The pill-box mounting will be less advantageous for this purpose, since, if the object be completely buried in the cell, there will be less power of seeing it on any but its upper surface; a pin may be secured to the bottom of such a box, however, by gumming over it a piece of stout paper. Disks may easily be punched, also, out of sheet gutta percha of any convenient thickness; or short cylinders may be made, by cutting up the thick cords made for lathe-bands; these are all readily penetrated by a heated pin, and no further trouble is necessary to attach them to it. Protective wells may also be made by cutting off short pieces from a gutta percha tube, and attaching these to the disks by a gentle heat. For the reception of boxes or disks thus mounted on pins, a drawer should be provided with a thick cork bottom, into which the pin is to be inserted far enough to prevent risk of displacement.

125. Mounting Objects in Canada Balsam.-This method of mounting is suitable to a very large proportion of those objects, which are to be viewed by transmitted light, and whose texture is not affected by the loss of the aqueous fluid they may contain; and it has many advantages over the mounting of the like objects dry. For, in the first place, as it fills up the little inequalities of their surface, even where it does not actually penetrate their substance, it increases their transparence by doing away with irregular refractions of the light in its way through them,

tents of each box, whilst protected and kept together, are at the same time presented to the eye. The Author has found the smallest and shallowest boxes of this kind that can be made, especially when lined with black paper, extremely useful for keeping Foraminifera and other Microscopic organisms in quantities; and also for mounting larger specimens for the Microscope as above described.

and gives them the aspect of perfect smoothness; this is well seen in the case of sections of Shell, &c., which, when thus mounted, do not require a high polish (§ 110). But, secondly, where the structure, although itself hard, is penetrated by internal vacuities, the balsam, by filling these, prevents that obscuration resulting from the interposition of air-spaces, and from additional internal surfaces of reflection, by which the transmitted rays are distorted, and a large proportion of them lost: this is well seen in the case of the Foraminifera, and of sections of the "test" and "spines" of Echinida, whose intimate structure can be far better made out, when they are thus mounted, than when mounted dry, although their substance is (for the most part at least) itself so dense, that the balsam cannot be imagined to penetrate it; and likewise with dry Vegetable preparations, which are perhaps also affected in the manner to be next described. Thirdly, there are very many structures of great interest to the Microscopist, whose appearance is extraordinarily improved by this method of mounting, in consequence of a specific effect which the balsam has in combining (so to speak) with their component elements, so as to render them far more transparent than before: this effect is seen in the case of all dry preparations of Insect structure, especially of such as consist of their hard external tegument or of parts derived from this; also in the various horny tissues (hairs, hoof, horn, &c.) of the higher animals; and likewise in many organized substances, both recent and fossil, which are penetrated by calcareous matter in an amorphous condition. Besides these advantages, the mounting of objects in Canada balsam affords one of the easiest methods of fixing and preserving them; and consequently, it is almost always had recourse to, in the case of such transparent objects as do not need to be preserved in fluid; save where, in virtue of the action just described, it impairs the distinctness of surface-markings, or obliterates internal cavities or canals, which constitute the most important features of the object.

126. Canada Balsam, being nothing else than a very pure Turpentine, is a natural combination of resin with the essential oil of turpentine. In its fresh state, it is a viscid liquid, easily poured out, but capable of being drawn into fine threads; and this is the condition in which the Microscopist will find it most desirable to use it for the mounting of objects generally. The balsam may be conveniently kept in a glass bottle or jar with a wide mouth, being taken up as required by a small glass rod drawn to a blunt point, such as is used by Chemists as a "stirrer;" and if, instead of a cork or stopper, this bottle should be provided with a tall hollow "cap," the glass rod may always stand in the balsam, with its upper end projecting into the cap. In taking out the balsam, care should be taken not to drop it prematurely from the rod; and not to let it come into contact with the interior of the neck or with the mouth of the jar. Both

these mischances may be avoided, by not attempting to take up on the rod more than it will properly carry; and by holding it in a horizontal position, after drawing it out from the bottle, until the slip on which it is to deposit the balsam, is just beneath its point. The Author has himself been in the habit of employing, instead of a hollow cap to his balsam jar, a disk of light wood, simply lying on its mouth; the centre of this disk is perforated with a hole, just large enough to allow the glass rod to fit it rather stiffly; and the rod is to be passed into the jar, only so far as to dip by its point into the balsam, being pushed further down as the level of the balsam is lowered. In this manner, a small quantity of balsam may be taken up with far less risk of messing it, than when the rod dips into it for two or three inches; if due care be taken not to allow the balsam to come into contact with the lip of the jar, the cover never sticks to it, but is readily lifted off upon the rod; and although it might seem an incumbrance in the use of the rod, yet it is not practically found to be so. If the balsam be kept too long, it becomes, through the loss of part of its volatile oil, too stiff for convenient use, and may be thinned by mixing it at a gentle heat with pure oil of turpentine; this mixture, however, does not produce that thorough incorporation of the constituents which exists in the fresh balsam; and it is consequently preferable to use in other ways the balsam which has become somewhat too stiff, and to have recourse to a fresh supply of liquid balsam for mounting purposes. When Canada balsam is to be employed as a cement, as for attaching sections, &c., to glass slides (§ 109), it should be in a much stiffer condition; since, if it be dropped on the slide in too liquid a state, it will probably spread much wider, and will lie in a thinner stratum than is desirable. This hardening process may be carried to any extent that may be desired, by exposing the balsam in an uncorked jar (the mouth of which however, should be covered with paper, for the sake of keeping off dust) to a continual gentle heat, such as that of a waterbath.

127. In mounting objects in Canada balsam, it is convenient to be provided with certain simple instruments, the use of which will save much time and trouble. For the heat required, a

FIG. 61.

Slider Forceps.

Spirit lamp is by far the best source; both as admitting of easy regulation, and as being perfectly free from smoke. Where a number of objects are being mounted on the same occasion, it will be found convenient to employ either a water-bath covered