swer the same puposes. In carrying on dissections in such a trough, it is frequently desirable to concentrate additional light upon the part which is being operated on, by means of the smaller condensing lens (Fig. 45); and when magnifying power is wanted, it may be supplied either by a single lens, mounted after the manner of Ross's Simple Microscope (Fig. 14, B), or by a Compound body mounted as in one of Mr. Warington's arrangements (Fig. 24). Portions of the body under dissection, being floated off when detached, may be conveniently taken up from the trough by placing a slip of glass beneath them (which is often the only mode in which delicate membranes can be satisfactorily spread out); and may be then placed under the microscope for minute examination, being first covered with thin glass, beneath the edges of which is to be introduced a little of the liquid wherein the dissection is being carried on. Where the body under dissection is so transparent, that more advantage is gained by transmitting light through it, than by looking at it as an opaque object, the trough should have a glass bottom; and for this purpose, unless the body be of unusual size, some of the glass "cells" to be hereafter described (§§ 136, 137) will usually answer very well. The finest dissections may often be best made upon ordinary slips of glass; care being taken to keep the object sufficiently surrounded by fluid. For work of this kind, no simple instrument is more generally serviceable than Mr. Quekett's Dissecting Microscope (Fig. 17); but if higher magnifying powers be needed than this will conveniently afford, recourse may be had to Smith and Beck's Dissecting Microscope (Fig. 29), which for this purpose should always be furnished with the Erector (Fig. 32). A particular arrangement of the light, devised many years since by the Author, will enable an expert dissector to prosecute his work with the naked eye, to an extent for which a lens would otherwise be required. This consists in giving to the object the same kind of black-ground illumination, as is now in common use for a very different purpose; and nothing more is necessary to afford it, than to attach to the under side of the stage a sort of "well," composed of a tube blackened in its interior, about 1 inch long, of the same diameter as the opening of the stage-plate, into the lower extremity of which a diaphragm or a ground-glass may be fitted, for the purpose of diminishing or of softening the light. The slide being laid upon the stage, and the mirror being so turned as to illuminate the object, the eye is to be so placed (the arm carrying the magnifiers being turned to one side) that the object is seen against the dark background afforded by the side of the well. In this manner, fibres of extreme minuteness, or other particles of extraordinary delicacy, can be clearly distinguished, such as could otherwise be scarcely discerned at all without the assistance of a magnifier. And the further the dissection can be carried in this mode, the less difficulty will be found in completing it, when the simple or

compound Microscope is brought to bear upon it. Whenever a dissection is being made upon the stage of a microscope, it is desirable that support should be provided for the hands on either side. This may be given by books or blocks of wood piled up to the requisite height; but in place of flat "rests," it is much more convenient to provide a pair of inclined planes, sloping away from the stage at an angle of about 30° below the horizon. These may be either solid blocks of wood, or (which is much less cumbrous) they may be made of two boards hinged together, one giving the inclined plane, which rests at one end upon the table, while the other, standing vertically, affords the requisite elevation to the extremity which abuts against the stage.

105. The instruments used in Microscopic dissection, are for the most part of the same kind as those which are needed in ordinary minute Anatomical research, such as scalpels, scissors, forceps, &c.; the fine instruments used in operations upon the eye, however, will commonly be found most suitable. A pair of delicate scissors curved to one side, is extremely convenient for cutting open tubular parts; these should have their points blunted; but other scissors should have fine points. A pair of fine-pointed scissors (Fig. 56), one leg of which is fixed in a light handle, and the other kept apart from it by a spring, so as to close by the pressure of the finger and to open of itself, will be found (if the blades be well sharpened on a hone) much superior to any

FIG. 56.

Spring-Scissors.

kind of knives, for cutting through delicate tissues with as little disturbance of them as possible: Swammerdam is said to have made great use of this instrument in his elaborate insect-dissections. Another cutting instrument much used by some dissectors, may be designated as a miniature of the shears used in shearing sheep, or as a cutting-forceps; the blades of such an instrument may be prevented from springing too far asunder, by means of a regulating-screw (as in the "microtome" of M. Strauss-Durckheim) or by some other kind of check; and the cutting action, being executed by the opposed pressure of the finger and thumb, may be performed with great precision. A pair of small straight forceps, with fine points, and another pair of curved forceps, will be found useful in addition to the ordinary dissecting-forceps. Of all the instruments contrived for delicate dissections, however, none are more serviceable than those which the Microscopist may make for himself out of ordinary needles. These should be fixed in light wooden handles' (the cedar sticks used for

1 Special needle-holders (like miniature port-crayons) have been made for this purpose: and although they afford the facility of lengthening or shortening the acting point of the needle at will, and also of carrying a reserve store of needles at the other end, yet the Author would decidedly recommend the use of the wooden handles, of which a large stock may be obtained for a trifle.

camel-hair pencils, or the handles of steel-pen-holders, will answer extremely well), in such a manner that their points should not project far, since they will otherwise have too much "spring" much may be done by their mere tearing action; but if it be desired to use them as cutting instruments, all that is necessary is to give them an edge upon a hone. It will sometimes be desirable to give a finer point to such needles, than they originally possess; this also may be done upon a hone. A needle with its point bent to a right angle, or nearly so, is often useful; and this may be shaped by simply heating the point in a lamp or candle, giving to it the required turn with a pair of pliers, and then hardening the point again by reheating it and plunging it into cold water or tallow.

106. Cutting Sections of Soft Substances.-Most important information respecting the structure of many substances, both Animal and Vegetable, may be obtained by cutting sections of

FIG. 57.

them, thin enough to be viewed as transparent objects. Where the substances are soft, no other instrument is necessary for this purpose, than a sharp knife, which may be best made with a thin two-edged blade like that of a lancet; considerable practice is needed, however, to make effectual use of it; and some individuals acquire a degree of dexterity, which others never succeed in attaining. In cutting sections of Animal tissues, which, owing to the quantity of water they contain, do not present a sufficiently firm resistance, it is often desirable to half-dry these, by exposing small pieces freely to the air, with the aid of a gentle warmth if required; when this desiccating process has been carried sufficiently far, thinner sections can be cut, than could possibly have been made in the original state of the tissue; and the texture, after a short maceration in water, almost entirely recovers its pristine characters. There are certain tissues, however, which will not bear to be thus treated, and of which it is sufficient to examine an extremely minute portion; and for making sections of these, such a pair of scissors as is represented in Fig. 57 will often be found very useful; since, owing to the curvature of the blades, the two extremities of a

Curved Scissors for

cutting Thin Sections.

The following is the mode in which the Author has found it convenient to mount his needles for this and other purposes-The needle being held firmly in a pair of pliers grasped by the right hand, its point may be forced into the end of a cedar or other stick held in the left, until it has entered to the depth of half an inch or more; the needle is then cut off to the desired length (the eye end being thus got rid of); and being then drawn out, the truncated end is forced into the hole previously made by the point, until it cannot be made to penetrate further, when it will be found to be very securely fixed. The end of the handle which embraces it, may then be bevelled away round its point of insertion.

2 It is difficult to convey by a drawing the idea of the real curvature of this instrument, the blades of which, when it is held in front view, curve-not to either side

section taken from a flat surface will generally be found to thin away, although the middle of it may be too thick to exhibit any structure. Where only a moderate degree of thinness is required, either in consequence of the transparence of the tissue, or because it is not desired to exhibit its minutest details, the two-bladed knife contrived by Prof. Valentin (Fig. 58) may be employed with advantage. The blades are attached to each other at their lower end by a screw, in such a manner that their "spring" tends to keep them apart; and their distance is regulated by pushing the little rivet backwards or forwards in the

FIG. 58.

Valentin's Knife.

slit through which it works. The knife should be dipped in water before using, or, still better, the section should be made under water, as the instrument works much better when wet; after use, it should be carefully washed and dried, a piece of soft leather being passed between the blades. If any water have found its way into the part through which the rivet works, the movable blade should be detached by taking out its screw, and each blade should be cleaned separately.'

107. Cutting Sections of Harder Substances.-There is a large class of substances, both Animal and Vegetable, which are too hard to admit of sections being made in the manner just described, but of which extremely thin slices can be made by a sharp cutting instrument, if only they be properly held and supported, more especially when the thickness of the section can be regulated by a mechanical contrivance; such are, in particular, the Stems and Roots of Plants, and the Horns, Hoofs, Cartilages, and similarly firm structures of Animals. Various costly machines have been devised for this purpose, some of them characterized by great ingenuity of contrivance and beauty of workmanship; but every purpose to which these are adapted, will be found to be answered by a very simple and unexpensive little instrument, which may either be held in the hand, or (which is preferable) may be firmly attached by means of a T-shaped piece of wood (as in Fig. 59), to the end of a table or work-bench.

but towards the observer; these scissors being, as the French instrument-makers say, courbés sur le plat. As an example of the utility of such an instrument to the Microscopist, the Author may cite the curious demonstration given a few years since, by Dr. Aug. Waller, of the structure of the gustative papillæ, by snipping off the papillæ from the living human tongue, which may be done with no more pain than the prick of a pin would occasion.

An improved form of this instrument is constructed by Mr. Matthews of Portugal Street; the blades being made with a convex instead of a straight edge, their distance from each other being regulated by a milled-head screw, and their separation for clean. ing being more easily accomplished.

This instrument essentially consists of an upright hollow cylinder

FIG. 59.

Section Instrument.

of brass, with a kind of piston which is pushed from below upwards by a fine-threaded screw, turned by a large milled head; at the upper end, the cylinder terminates in a brass table, which is made to present a perfectly flat surface. At one side is seen a small milled head, which acts upon a "bindingscrew," whose extremity projects into the cavity of the cylinder, and serves to compress and steady anything that it holds. A cylindrical stem of wood, a piece of horn, whalebone, cartilage, &c., is to be fitted to the interior of the cylinder, so as to project a little above its top, and is to be steadied by the "binding-screw;" it is then to be cut to a level by means of a sharp knife or razor, laid flat upon the table. The milled head is next to be moved through such a portion of a turn, as may very slightly elevate the substance to be cut, so as to make it project in an almost insensible degree above the table; and this projecting part is to be sliced off with a knife, previously dipped in water. The best knife for this purpose is a razor, ground flat (instead of concave) on one side, but having still a concave surface on the other; the flat side is to be laid downwards upon the table; and the motion given to the edge should be a combination of drawing and pressing. (It will be generally found that better sections are made, by working the knife from the operator, than towards him.) When one slice has been thus taken off, it should be removed from the blade by dipping it into water, or by the use of a camel-hair brush; the milled head should be again advanced, and another section taken; and so on. Different substances will be found both to bear and to require different degrees of thickness; and the amount that suits each can only be found by trial. It is advantageous to have the large milled head graduated, and furnished with a fixed index; so that this amount having been once determined, the screw shall be so turned as to always produce the exact elevation required. Where the substance of which it is desired to obtain sections by this instrument, is of too small a size or of too soft a texture to be held firmly in the manner just described, it may be placed between the two vertical halves of a cork of suitable size to be pressed into the cylinder; and the cork, with the object it grasps, is then to be sliced in the manner already described, the small section of the latter being carefully taken off the knife, or floated away from it, on each occasion, to prevent it from being