SECTION V.. _on the Photographic Delineation of Microscopic Objects._

On the Photographic delineation of Microscopic objects.

Many specimens of Micro-photography which have been exhibited are exceedingly elaborate and beautiful; and their production is not difficult to one thoroughly acquainted with the use of the Microscope and with the manipulations of the Collodion Process. It is important however to possess a good apparatus, and to have it properly arranged.

The object-glass of the ordinary compound Microscope is the only part actually required in Photography, but it is useful to retain the body for the sake of the adjustments, and the mirrors used in the illumination. The eye-piece however, which simply magnifies the image formed by the object-glass, is not necessary, since the same effect of enlargement may be obtained by lengthening out the dark chamber, and throwing the image further off.

Arrangement of the Apparatus.--The Microscope is placed with its body in a horizontal position, and the eye-piece being removed, a tube of paper, properly blackened in the interior, or lined with black velvet, is inserted into the instrument, to prevent irregular reflection of light from the sides.

A dark chamber of about two feet in length, having at one end an aperture for the insertion of the eye-piece end of the body, and at the other a groove for carrying the slide containing the sensitive plate, is then attached; care being taken to stop all crevices likely to admit diffused light. An ordinary Camera may be employed as the dark chamber, the lens being removed, and the body lengthened out if required by a conical tube of gutta-percha, made to fasten into the flange of the lens in front. The whole apparatus should be placed exactly in a straight line, that the ground glass used in focussing may fall at right angles to the axis of the Microscope.

The length of the chamber, measuring from the object-glass, may be from two to three feet, according to the size of image required; but if extended beyond this, the pencil of light transmitted by the object-glass is diffused over too large a surface, and a faint and unsatisfactory picture is the result. The object should be illuminated by sunlight if it can be obtained, but a bright diffused daylight will succeed with low-power glasses, and especially when Positives are taken. Employ the concave mirror for reflecting the light on the object in the latter case; but in the former the plane mirror is the best, except with powers exceeding a quarter of an inch, and of large angular aperture.

The image upon the ground glass should appear bright and distinct, and the field of a circular form and evenly illuminated; when this is the case, all is ready for inserting the sensitive plate.

The time of exposure must be varied according to the intensity of the light, the sensibility of the Collodion, and the degree of magnifying power; a few seconds to a minute will be about the extremes; but minute directions are not required, as the operator, if a good Photographer, will easily ascertain the proper time for exposing (see page 224).

At this point a difficulty will probably occur from the plane of the chemical focus not corresponding, as a rule, with that of the visual focus. This arises from the fact that the object-glasses of Microscopes are "over-corrected" for colour, in order to compensate for a little chromatic aberration in the eye-piece. The violet rays, in consequence of the over-correction, are projected beyond the yellow, and hence the focus of chemical action is further from the glass than the visible image.

The allowance may be made by shifting the sensitive plate, or, what amounts to the same thing, by removing the object-glass a little away from the object with the fine adjustment screw; the latter is the most convenient. The exact distance must be determined by careful experiment for each glass; but it is greatest with the low powers, and decreases as they ascend.

Mr. Shadbolt gives the following as a guide:--"An inch and a half objective of Smith and Beck's make required to be shifted 1/50th of an inch, or two turns of their fine adjustment; a 2/3rds of an inch, 1/200th of an inch, or half a turn; and a 4/10ths of an inch, 1/1000th of an inch, or about two divisions of the adjustment. With the 1/4th and higher powers, the difference between the foci was so small as to be practically unimportant."

There is also reason to think that the kind of light employed has an influence upon the separation of the foci. Mr. Delves finds that with sunlight the difference between them is very small even with the low powers, and inappreciable with the higher; whereas in using diffused daylight which has undergone a previous reflection from white clouds, it is considerable.

The object-glasses of the same maker, and particularly those of different makers, also vary much; so that it will be necessary to test each glass separately, and to register the allowance which is required.

Having found the chemical focus, the principal difficulty has been overcome, and the remaining steps are the same in every respect as for ordinary Collodion Photographs.

To those who cannot devote their time to Photography during the day, Mr. Shadbolt's observations on the use of artificial light may be of service. He employs Camphine, which gives a whiter flame than gas, or a moderator lamp; placing the source of light in the focus of a plano-convex lens of 2-1/2 to 3 inches diameter (the flat side towards the lamp), and condensing the parallel rays so obtained on the object, by a second lens of about 1-1/2-inch diameter and 3-inch focus.

This mode of illumination, being feeble in chemical rays, is best adapted for object-glasses of low power. The exposure required to produce a Negative impression with the one-inch glass may be from three to five minutes. As the sensitive plate would be liable to become dry during that time, it is recommended to coat it with some preservative solution by the modes described in the sixth Chapter. Mr. Crookes having lately shown that the Bromide of Silver is more sensitive than the Iodide to artificial light, a mixture of the two salts may conveniently be used (see pp. 66 and 232).

The development may be conducted in the same manner as that for preserved sensitive plates; fixing with Cyanide of Potassium before the development is fully complete, if any tendency to fogging is observed (see page 224).

The Rev. W. Towler Kingsley has communicated a process by which very beautiful Microscopic Photographs have been obtained. He illuminates (in the absence of sunlight) with the brilliant light produced by throwing a jet of mixed Oxygen and Hydrogen gases upon a small cone of Lime or Magnesia. Particular stress is laid upon the object-glass of the Microscope being a good one for the purpose; and indeed all who have given attention to the subject are agreed upon this point--that there is a considerable difference in the Photographic value of objectives, and this independent of the angular aperture of the glass.