Workers with the Microscope are generally desirous of obtaining pictorial records of many of the objects whose structure that wonderful instrument allows them to examine. Hitherto such records have been made with the pencil, by the aid of an attachment to the Microscope, known as the Camera lucida. But although the artist may be conscientious as well as skilful, it is next to impossible for him to obtain a really trustworthy representation of what he tries to copy. This is because a draughtsman cannot help investing his work with a certain individuality; so that although two men might try their best to faithfully copy the same object, the results would show a very great amount of dissimilarity. We all know that a man's handwriting is something peculiar to him, and that its particular traits will become evident even if he tries to write in a feigned manner. So it is with an artist's pencil, a circumstance which enables experts to detect the work of different known hands with unfailing accuracy.
Now it is evident that in the pourtrayal of Microscopic objects, where truth is the one thing needful, and where artistic touches are not required; the photographic plate can give a more correct result than any mere drawing. This is now conceded by the best workers, and Photo-Micrography as it is called, has become a branch of science in which many excel. With these preliminary remarks, we will proceed to give such detailed directions as will enable those who are desirous of doing so, to succeed in this very interesting and instructive field of photographic work.
First of all, let us say a few words with regard to the type of Microscope to be employed. It need not be an expensive one; but it is advisable that it should possess a circular revolving stage having mechanical adjustments for centreing the object. It must be firm, and on some description of foot which cannot readily be knocked over. It must be so constructed that the entire instrument with the exception of the foot can be bent down to the horizontal position. Its tube should be short and thick. It must have a coarse adjustment, regulated by rack and pinion in the usual way, and also a fine adjustment. This latter should be so conveniently placed on the instrument, and its milled head cut with a groove, that it can be turned by means of an attached cord, in the manner to be presently described. The stage upon which the objects are placed, should be a revolving one, with brass clips to hold the slide firmly in position when the instrument is placed horizontally. Beneath the stage there should be an internal screw to receive a condenser. And it may be mentioned here that an achromatic object glass of the triple (French) form answers the purpose well. But care should be taken that the power of this condenser should never exceed the power of the objective in use. The French triplet, consisting of three glasses, each mounted separately, but screwing together, admits of regulation in this respect. For instance, in using the quarter inch objective, two of the French ones would be employed. But if the eighth objective were in use, then the complete triplet would form a suitable sub-stage, condenser. With the inch power no condensing lens on the sub-stage would be necessary.
The ordinary lens of the camera is removed, and a short tube (lined with black velvet, and of such a diameter that the microscope tube will easily slide within it) is put in its place. The eyepiece of the microscope having been removed, the tube is slipped into the velvet lined aperture just described. The microscope tube should also be lined with velvet, or reflected light is sure to do some mischief when photography is commenced. The lamp is now placed behind the stage, as shown in page 128, and must be very carefully adjusted to the correct height. In front of it is placed the condenser, with its convex face towards the camera. A low power objective, say a one inch, is now screwed on to the microscope, the lamp is lighted, and the ground glass focussing screen of the camera examined. If every part of the apparatus has been correctly centred, the screen should exhibit a clear disc of light. If one portion appears to be brighter than another, it is quite certain that something is out of centre. The lamp may possibly be a little out of adjustment, or the condenser perhaps wants to be moved a trifle. A friendly assistant will be of great use here in trying the various adjustments while the operator, covered with the focussing cloth, is watching the screen. Until a clear disc is obtained, the work cannot proceed further.
When all seems to be perfect in the above respects, an object can be placed upon the stage and roughly focussed with the coarse adjustment. This object should be some well known one, so that the operator may know how it should appear, and what to look for. And now we must use the fine adjustment. It has been already pointed out that the milled head of this latter should be provided with a groove. In this groove is slipped a silk cord which is geared to a pulley wheel upon a long focussing rod at the side of the table (see page 128. The other end of this rod is furnished with a button which is within reach of, and can readily be turned by the hand of the operator as he watches the focussing screen.
In a photographic Jens the visual and chemical foci are made to coincide, so that a picture which appears to be sharply defined on the camera screen will give a sharp picture when photographed. But in the microscope objective this is not always the case, and more especially in the case of low powers will this difference of foci become apparent. So that an image which appears to be sharp enough on the screen, turns out to be indistinct in the negative. Where this difficulty arises, it can be corrected in the following manner. After the image appears to be sharply defined, turn the fine adjustment so as to make the objective approach towards the object until the image on the focussing screen appears to be surrounded by a red areola. Although at this point it may not seem to be as sharply defined as before, the resulting negative will be all right.