The Preparation & Mounting of Microscopic Objects

by microscopist Thomas Davies

I may here mention that as this manual is simply to enable the young student to prepare and mount his objects, the photography of magnified objects has evidently no place here.

Few slides caused so much astonishment as these minute photographs when first exhibited; small spots were seen to contain large pictures, and a page of printed matter was compressed into the one-hundredth part of a square inch. It would be impossible in this place to give the inquirer any instruction in the manipulation of photography, so it must be assumed that he already knows this.

We will first consider the process performed by artificial light. The collodion employed in photographing generally shows as much structure when magnified as is found in linen of moderate texture; but this is not always the case, as some samples bear much enlargement without any of this appearance. It is evident that a structure so coarse would make it entirely unfit for these minute pictures, as all the small markings would be destroyed, or so interfered with that no great enlargement would be practicable. To obtain almost structureless collodion is not an easy matter, and a clever practitioner in this branch of photography states that he knows of no method to accomplish this with certainty, but he himself tries different samples until he falls upon a suitable one, which he then lays aside for this object. A beneficial effect is often derived from keeping the collodion awhile, but this is not always the case. The slides should be chosen of an equal thickness, so that when focussed upon one no re-adjustment may be necessary for the others. The glass should, of course, be free from any roughness, scratches, or other imperfections, and of first-rate quality and colour.

The microscope must then be placed in a horizontal position, and the eye-piece removed, the stage having a small clip upon it to keep the prepared plate in position. The negative must then be supported at a distance from the end of the microscope tube from which the eye-piece was withdrawn. This distance will, of course, vary according to the relative sizes of the negative and desired picture. With a one-inch object-glass, which is a very convenient focus, it will have to be changed usually betwixt one and four feet. The negative must be lighted by an argand gas-burner or camphine-lamp, and the rays rendered as parallel as possible by the use of a large lens placed betwixt the light and the negative. It is not easy to arrange the apparatus so as to get the light uniform; but a little practice will soon do away with this difficulty. Ordinary ground-glass is too coarsely grained to focus upon, as the magnifying power used to examine the minute reflection must be considerable. One of the slides must therefore be coated with the collodion, submitted to the silver-bath, and after washing with water be allowed to dry. Upon this may be focussed the reflected image, and its minuteness examined with a powerful hand-magnifier, or another microscope placed behind in a horizontal position. When the utmost sharpness of definition is obtained, it is usually required to remove the plate a little distance from the object-glass, as object-glasses for the microscope are slightly “over corrected,” and the chemical rays which accomplish the photography are beyond the visual ones. The exact distance required to give a picture to bear the greatest enlargement cannot be given by rule; but experiments must be made at first, and it will always be the same with the object-glass which we have tested.

The plate may now be prepared as in ordinary photography, and placed upon the stage whilst the light is shaded. When all is ready, the shade is removed and the process allowed to go on, usually for thirty or forty seconds; but no certain rule can be given as to the required time, on account of various collodions, lamps, and powers being used. It may be here mentioned, that it is well to contrive some little frame to receive the prepared plate, as the silver bath solution is liable to get upon the microscope stage and so, to say the least, disfigure it. When the exposure has been continued sufficiently long, the picture may be developed by any of the ordinary methods, but some of the best productions have been brought out by the aid of pyrogallic and citric acid solution, with the addition of a little alcohol. The “fixing” may be effected by a strong solution of hypo-sulphite of soda, and the picture must then be very well washed with pure water. When dry, the photograph must be mounted with Canada balsam, in the same manner as any ordinary object; but great heat must not be used, or the picture may be injured.

When ordinary daylight is employed for this purpose, a dark slide will be required for the prepared plate, in the same way as for photographing landscape, &c. These dark slides are generally made by each individual to suit his particular arrangements of negatives, &c.; but it may be here recommended that the operator should always focus in the same slide which he is about to use, as so small a difference in distance lies betwixt perfection and failure.

For an ordinary student, perhaps the above method is that which is the most readily used, and consequently the most generally available; but almost every one has a different arrangement of microscope, &c., by which he procures these minute pictures. Mr. Shadbolt (one of our most successful photographers) gives the following instructions:—“Having removed the upper stage plate of a large compound microscope, I replace it with one of wood, supplied with guide pins of silver wire, in order to admit of its supporting a slip of glass coated with collodion, and excited in the nitrate of silver bath in the usual way. If the ordinary brass stage plate were left undisturbed, it is obvious that it and the excited slip of glass would be mutually destructive.

“The microscope is now to be placed in a horizontal position, the objective, intended to produce the picture, made to occupy the place usually filled by the achromatic condenser on the sub-stage of the microscope, while another objective is screwed into the lower end of the body of the instrument, which is used not only to focus with, but also to make the requisite allowance for actinic variation.

“The negative intended to be reduced is then arranged vertically, with its centre in the axis of the microscopic body, at a distance of from two to four feet from the lower object-glass, and with a convenient screen of card, wood, or thick paper, to cut off any extraneous light that would otherwise pass beyond the limits of the picture.

“A small camphine-lamp is employed for the purpose of illuminating the negative, having a good bull’s-eye lens as a condenser, so arranged with its flat side next the lamp that the refracted rays shall just fill the whole of a double convex lens of about six inches in diameter, the latter being placed in such a position as to refract the rays of light in a parallel direction upon the negative. By this arrangement the bull’s-eye lens of about two inches and a half in diameter appears as the source of the light instead of the small flame of the lamp.