CHAPTER II.
TO PREPARE AND MOUNT OBJECTS “DRY.”
The term “dry” is used when the object to be mounted is not immersed in any liquid or medium, but preserved in its natural state, unless it requires cleaning and drying.
I have before stated that thorough cleanliness is necessary in the mounting of all microscopic objects. I may here add that almost every kind of substance used by the microscopist suffers from careless handling. Many leaves with fine hairs are robbed of half their beauty, or the hairs, perhaps, forced into totally different shapes and groups; many insects lose their scales, which constitute their chief value to the microscopist; even the glass itself distinctly shows the marks of the fingers if left uncleaned. Every object must also be thoroughly dry, otherwise dampness will arise and become condensed in small drops upon the inner surface of the thin glass cover. This defect is frequently met with in slides which have been mounted quickly; the objects not being thoroughly dry when enclosed in the cell. Many of the cheap slides are thus rendered worthless. Even with every care it is not possible to get rid of this annoyance occasionally.
For the purpose of mounting opaque objects “dry,” discs were at one time very commonly made use of. These are circular pieces of cork, leather, or other soft substance, from one-quarter to half inch in diameter, blackened with varnish or covered with black paper, on which the object is fixed by gum or some other adhesive substance. They are usually pierced longitudinally by a strong pin, which serves for the forceps to lay hold of when being placed under the microscope for examination. Sometimes objects are affixed to both sides of the disc, which is readily turned when under the object-glass. The advantage of this method of mounting is the ease with which the disc may be moved, and so present every part of the object to the eye save that by which it is fastened to the disc. On this account it is often made use of when some particular subject is undergoing investigation, as a number of specimens may be placed upon the discs with very little labour, displaying all the parts. But where exposure to the atmosphere or small particles of dust will injure an object, no advantage which the discs may possess should be considered, and an ordinary covered cell should be employed. Small pill-boxes have been used, to the bottom of which a piece of cork has been glued to afford a ground for the pin or other mode of attachment; but this is liable to some of the same faults as the disc, and it would be unwise to use these for permanent objects.
Messrs. Smith and Beck have lately invented, and are now making, a beautiful small apparatus, by means of which the disc supporting the object can be worked with little or no trouble into any position that may prove most convenient, whilst a perforated cylinder serves for the reception of the discs when out of use, and fits into a case to protect them from dust. A pair of forceps is made for the express purpose of removing them from the case and placing them in the holder.
All dry objects, however, which are to be preserved should be mounted on glass slides in one of the cells (described in [Chapter I.]) best suited to them. Where the object is to be free from pressure, care must be taken that the cell is deep enough to ensure this. When the depth required is but small, it is often sufficient to omit the card, leather, or other circles, and with the “turn-table” before described, by means of a thick varnish and camel-hair pencil, to form a ring of the desired depth; but should the varnish not be of sufficient substance to give such “walls” at once, the first application may be allowed to dry, and a second made upon it. A number of these may be prepared at the same time, and laid by for use. When liquids are used (see [Chapter IV.]), Dr. Carpenter recommends gold-size as a good varnish for the purpose, and this may be used in “dry” mountings also. I have used the asphaltum and india-rubber (mentioned in [Chapter I.]), and found it to be everything I could wish. The cells, however, must be thoroughly dry, and when they will bear the heat they should be baked for an hour at least in a tolerably cool oven, by which treatment the latter becomes a first-rate medium. All dry objects which will not bear pressure must be firmly fastened to the slide, otherwise the necessary movements very often injure them, by destroying the fine hairs, &c. For this purpose thin varnishes are often used, and will serve well enough for large objects, but many smaller ones are lost by adopting this plan, as for a time, which may be deemed long enough to harden the varnish, they exhibit no defect, but in a while a “wall” of the plastic gum gathers around them, which refracts the light, and thus leads the student to false conclusions. In all finer work, where it is necessary to use any method of fixing them to the slide, a solution of common gum, with the addition of a few drops of glycerine ([Chapter I.]), will be found to serve the purpose perfectly. It must, however, be carefully filtered through blotting paper, otherwise the minute particles in the solution interfere with the object, giving the slide a dusty appearance when under the microscope.
When mounting an object in any of these cells, the glass must be thoroughly cleaned, which may be done with a cambric handkerchief, after the washing mentioned in [Chapter I.] If the object be large, the point of a fine camel-hair pencil should be dipped into the gum solution, and a minute quantity of the liquid deposited in the cell where the object is to be placed, but not to cover a greater surface than the object will totally hide from sight. This drop of gum must be allowed to dry, which will take a few minutes. Breathe then upon it two or three times, holding the slide not far from the mouth, which will render the surface adhesive. Then draw a camel-hair pencil through the lips, so as to moisten it slightly (when anything small will adhere to it quite firmly enough), touch the object and place it upon the gum in the desired position. This must be done immediately to ensure perfect stability, otherwise the gum will become at least partially dry and only retain the object imperfectly.
When, however, the objects are so minute that it would be impossible to deposit atoms of gum small enough for each one to cover, a different method of proceeding must be adopted. In this case a small portion of the same gum solution should be placed upon the slide, and by means of any small instrument—a long needle will serve the purpose very well—spread over the surface which will be required. The quantity thus extended will be very small, but by breathing upon it may be prevented drying whilst being dispersed. This, like the forementioned, should be then allowed to dry; and whilst the objects are being placed on the prepared surface, breathing upon it as before will restore the power of adherence.
When gum or other liquid cement has been used to fix the objects to the glass, the thin covers must not be applied until the slide has been thoroughly dried, and all fear of dampness arising from the use of the solution done away with. Warmth may be safely applied for the purpose, as objects fastened by this method are seldom, if ever, found to be loosened by it. As objects are met with of every thickness, the cells will be required of different depths. There is no difficulty in accommodating ourselves in this—the deeper cells may be readily cut out of thick leather, card, or other substance preferred (as mentioned in [Chapter I.]). Cardboard is easily procured of almost any thickness; but sometimes it is convenient to find a thinner substance even than this. When thin glass is laid upon a drop of any liquid upon a slide, every one must have observed how readily the liquid spreads betwixt the two: just so when any thin varnish is used to surround an object of little substance, excessive care is needed lest the varnish should extend betwixt the cover and slide, and so render it worthless. The slightest wall, however, prevents this from taking place, so that a ring of common paper may be used, and serve a double purpose where the objects require no deeper cell than this forms.
Many objects, however, are of such tenuity—as the leaves of many mosses, some of the Diatomaceæ, scales of insects, &c.—that no cell is requisite excepting that which is necessarily formed by the medium used to attach the thin glass cover to the slide; and where the slide is covered by the ornamental papers mentioned in [Chapter I.], and pressure does not injure the object, even this is omitted, the thin glass being kept in position by the cover; but slides mounted in this manner are frequently injured by dampness, which soon condenses upon the inner surfaces and interferes both with the object and the clearness of its appearance.