Page images
PDF
EPUB

main entire after the cutting of disks (§ 117) may be employed, the disks often falling out of themselves when the glass is laid aside for a few days; and thus the same piece of thin glass may afford a plate, which, when cemented to a glass slide forms a cell, and a disk suitable as the cover to a cell of somewhat smaller size. There is great danger, however, of the cracking of the surrounding glass, especially when the disk is of large size; and it will generally be found a saving of trouble, to employ the method recommended by Dr. L. Beale. This consists in attaching a piece of thin glass to one of the glass rings of which the deeper cells are made (§ 137), of any form that may be desired, by means of marine glue, first laid upon the latter, and melted upon the hot plate; when the glue is quite cold, the point of a round or semicircular file is sharply thrust through the centre of the thin glass, which is carefully filed to the size of the interior of the ring; and the ring being then heated a second time on the hot plate, the thin glass plate may be readily detached from it, and at once cemented upon the glass slide. The success of this simple process depends upon the very firm and intimate adhesion of the thin glass to the ring, which prevents any crack from running into the part of the thin glass that is attached to it, however roughly the file may be used. By having many of the rings on the hot plate at once, and operating with them in turn, a great number of cells can be made in a short time; and such large thin cells may be made in this mode, as could scarcely be fabricated (on account of the extreme brittleness of this glass) by any other. A press, consisting of two plates of brass screwed together, holding the thin glass between them, has been devised by Mr. C. Brooke for the same purpose; but the foregoing method has the advantage, not only of requiring no special apparatus, but also of enabling the form and size of the perforation to be readily varied. After the thin glass has been cemented to the slide, it is desirable to roughen its upper surface, by rubbing it upon a leaden or pewter plate (§ 108) with fine emery; since the gold-size or other varnish adheres much more firmly to a "ground" than to a polished surface. Although the thin glass cell requires much more trouble in its preparation than the cement cell, yet it is decidedly to be preferred for any very choice objects; since, if any air should find admission, it is more readily detected; and the remounting of the object may be accomplished in the same cell, with very little disturbance of its position.

136. Plate Glass and Shallow Cells.-For mounting objects of somewhat greater thickness than can be included within thin glass cells, shallow cells may be made by drilling apertures of the desired size in pieces of plate-glass of the requisite thickness, and by attaching these with marine glue to glass slides (Fig. 64). Such holes may be made not merely circular (A) but oval (c); a very elongated perforation may be made, by drilling two

and

holes at the required distance and then connecting them by cutting out the intermediate space (B). These operations, however, can scarcely be performed by any but regular glass-cutters, and, being troublesome, are expensive; hence the plate-glass cells have been generally superseded, either by tube-cells (§ 137) or by

FIG. 64.

Plate Glass Cells.

built-up cells. Although the former may be reduced to any degree of shallowness A that may be desired, and are made of most of the sizes and forms that can be ordinarily needed, yet for extra sizes or B peculiar forms, shallow cells may be easily built up after the following very simple and effective method. A piece of plate glass, of a thickness that c shall give the desired depth to the cell, is to be cut to the dimensions of its outside wall; and a strip is then to be cut off with the diamond from each of its edges, of such breadth as shall leave the in

D

terior piece equal in its dimensions to the cavity of the cell, that is desired. This piece being rejected, the four strips are then to be cemented upon the glass slide in their original position, so that the diamond cuts shall fit together with the most exact precision; and the upper surface is then to be ground flat with emery upon the pewter plate, and left rough as before. This plan answers admirably for constructing such large shallow cells as are required for the mounting of Zoophytes and similar objects.

Having had occasion, during the last few months, to mount a large number of objects in shallow cells, the Author has adopted the recommendation of a friend, to make use of cells which are sunk by grinding out a concave in the thickness of a glass plate. These, until recently, were costly; but they are now made in large quantities, and their price has been so much reduced, that they can be obtained more cheaply than any other kind. For objects whose shape adapts them to the form and depth of the concavity, these cells will be found peculiarly advantageous; since they do not hold air-bubbles so tenaciously as do those with perpendicular walls; and there is no cemented plate or ring to be loosened from its attachment, either by a sudden jar or by the lapse of time. For transparent objects, however, they are less suitable (unless manufactured with more care than is usually given to them) than they are for opaque; since the concave bottom is seldom so highly polished, as to be free from scratches and rough

nesses, which greatly interfere with the appearance of the picture. Cells of this kind may be obtained, from Messrs. Jackson, Oxford Street, either of round or oval form, and not only ground out of slides of the usual size (3 in. by 1) and thickness, but also hollowed in pieces of plate-glass of larger dimensions.

A

FIG. 65.

137. Deep and Built-up Cells.-The deep cells which are required for mounting Injections and other microscopic preparations of considerable size and thickness, may be made by drilling through a piece of thick plate glass (Fig. 64, D); but for the reason already given, the drilled cells are now seldom used, their place having been taken, either by tube-cells, or by the deep built-up cells to be presently described. The tube-cells are made by cutting transverse sections of thick-walled glass tubes of the required size, grinding the surfaces of these rings to the desired thinness, and then cementing them to the glass slides with marine glue. Not only may round cells (Fig. 65, A, B), of any diameter and any depth that the Microscopist can possibly require, be made by this simple method, but oval, square-shaped, or oblong cells (C, D) are now made, of the forms and sizes that he is most likely to want, by flattening the round glasstube whilst hot, or by blowing it within a mould. The facility with which such cells may be made, and the security they afford, have caused the deep cells built up of separate strips of glass c (Fig. 66) to be comparatively little employed, except in cases where some very unusual size or shape (A) may be required, or where it is D necessary that not merely the top and bottom, but also the sides of the object, should

B

Tube-Cells, Round and Quadrangular.

А

be clearly seen (B). The perfect construction of these requires a nicety of workmanship which few amateurs possess, and the expenditure of more time than Microscopists generally have to spare; and as it is consequently preferable to obtain them ready made, directions for making them need not here be given. Ä new plan of making deep cells, however, has been lately introduced by Dr. L. Beale; which, though it does not give them side walls possessing the same flatness with those of the built-up cells, adapts them to serve most of the purposes for which these are required, and makes them more secure against leakage; whilst it has the advantage of being so easy and simple, that

A long strip of plate glass is

any one may put it into practice. to be taken, whose breadth is equal to the desired depth of the

FIG. 66.

B

cell, and whose length must be equal to the sum of that of all its sides. This strip is to be carefully bent to a right angle A in the blow-pipe flame, at three points previously indicated by marks so placed as to show where the angles should fall; and the two ends, which will thus be brought into contact at right angles, are to be fused together. Thus a large square well, slightly rounded at the angles, will be formed; and this, being very brittle, should be allowed to cool very gradually, or, still better, should be annealed in an oven. It must then be ground quite true on its upper and lower edges, either on the lead plate with emery, or on a flat stone with fine sand; and it may then be cemented to a glass slide in the usual way.

Built-up Cells.

138. Mounting Objects in Cells.-In mounting an object in a cell, the first attention will of course be given to the cleanness of the interior of the cell, and of the glass cover which is to be placed on it; this having been secured, the cell is to be filled with fluid by the pipette or syringe; and any minute air-bubbles which may be seen adhering to its bottom or sides, must be removed by the needle: the object, previously soaked in fluid resembling that with which the cell is filled, is then to be placed in the cell, and should be carefully examined for air-bubbles on all sides, and also by looking up from below. When every precaution has been taken to free it from these troublesome intruders, the cover may be placed on the cell, one side being first brought down upon its edge and then the other; and if the cell have been previously brimming over with fluid (as it ought to be) it is not likely that any air-space will remain. If, however, any bubbles should present themselves beneath the cover, the slide should be inclined, so as to cause them to rise towards the highest part of its circumference, and the cover slipped away from that part, so as to admit of the introduction of a little additional fluid by the pipette or syringe; and when this has taken the place of the air-bubbles, the cover may be slipped back into its place. All superfluous fluid is then to be taken

1 Mr. Quekett and some other practised manipulators recommend that the edges of the cell and that of the disk of glass be smeared with the gold-size or other varnish employed, before the cell is filled with fluid; but the Author has found this practice objectionable, for two reasons-first, because it prevents the cover from being slipped to one side (which is often desirable), without its being soiled by the varnish,—and second, because when the edge of the cell has been thus made to "take" the varnish, that which is afterwards applied for the closure of the cell is more likely to run in, than if the whole of the surface covered by the glass is moistened with water.

up with blotting-paper; and particular care should be taken thoroughly to dry the surface of the cell and the edge of the cover, since the varnish will not hold to them if they be in the least damp with water. Care must also be taken, however, that the fluid be not drawn away from between the cover and the edge of the cell on which it rests; since any deficiency here is sure to be filled by varnish, the running in of which is particularly objectionable. These minutia having been attended to, the closure of the cell may be at once effected, by carrying a thin layer of gold-size or asphalte around and upon the edge of the glass cover, taking care that it touches every point of it, and fills the angular channel which is left around its margin. If the wall of the cell be very thin, it will be advantageous to include it in the ring of varnish; so that this shall hold down the cover, not only on the cell, but on the slide beneath. The Author has found it advantageous, however, to delay closing the cell for some little time after the superfluous fluid has been drawn off; for as soon as evaporation beneath the edges of the cover begins to diminish the quantity of fluid in the cell, air-bubbles often begin to make their appearance, which were previously hidden in the recesses of the object; and in the course of half an hour, a considerable number are often collected. The cover should then be slipped aside, fresh fluid be introduced, the air-bubbles removed, and the cover put on again; and this operation should be repeated, until it fails to draw forth any more air-bubbles. It will, of course, be observed, that, if the evaporation of fluid should proceed far, air-bubbles will enter beneath the cover; but these will show themselves on the surface of the fluid; whereas those which arise from the object itself, are found in the deeper parts of the cell. Much time may be saved, however, and the freedom of the preparation from air-bubbles may be most effectually secured, by placing the cell, after it has been filled in the first instance, in the vacuum of an air-pump; and if several objects are being mounted at once, they may all be subjected to the exhausting process at the same time. The application of the varnish should be repeated after the lapse of a few hours, and may be again renewed with advantage several times in the course of a week or two; care being taken that each layer covers the edges, as well as the whole surface, of that which preceded it.

139. The presence of air-bubbles, in any preparation mounted in fluid, is to be particularly avoided, not merely on account of its interference with the view of the object, but also because, when air-spaces, however small, once exist, they are almost certain to increase, until at last they take the place of the entire fluid, and the object remains dry. Even with the most experienced manipulators, however, this misfortune not unfrequently occurs; being sometimes due to the obstinate entanglement of air-bubbles in the object, when it was originally mounted;

« PreviousContinue »