On the Photographic Delineation of Microscopic Objects by Artificial Illumination

As it is not my intention to enter into particulars of the rise and progress of this art as connected with the microscope, I will only observe that the earliest microscopic photographs which I had the pleasure of seeing were some Daguerreotypes executed by Mr. Richard Hodgson by the aid of the direct rays of the sun ; and for these I believe he is entitled to claim the honour of having been the first to produce a picture of this kind.

But however beautiful the sharpness and detail of pictures upon metallic plates, there are many causes to confine the practice of the Daguerreotypic art within such very contracted limits as to render it of but little use to the microscopist; whereas the increasing beauty and sensibility of the Collodion process renders it a much more encouraging medium for further experiment in this direction, besides offering the additional inducement of enabling one to transmit duplicates upon paper to others engaged upon similar observations at a distant part, by which comparisons of much value can be made, and without the expense and inconvenience of having to execute duplicates from the objects themselves.

As it happens that the great majority of the followers of microscopic science are mostly engaged in professional or other business pursuits during the day-time, and in most instances at a distance from home, it occurred to me that if artificial light could be made to act sufficiently energetically to produce microscopic pictures, it would be a very considerable advantage to a large number of persons who would otherwise not be able to avail themselves of so excellent an assistant as the photographic art; and further, that to render it practically useful, it must be done by an illumination readily accessible. and inexpensive; I therefore determined to institute a series of experiments with this end in view, and having availed myself of all the hints thrown out by Mr. Delves, Mr. Hogg, and others, at the Microscopical meeting in October, after very many failures and no small amount of trouble, I at length was fortunate enough to meet with such success as, in my opinion, to offer very considerable encouragement for further operations with a reasonable hope of a really useful result; and at the meeting of the Microscopical Society in November last I bad the pleasure of exhibiting a picture of a Fly’s Proboscis, produced by the aid of a very small camphine lamp. In the hope of enlisting more labourers in this field of research, I purpose detailing the “ modus operandi “ which I have found most successful; trusting that, in a short time, the little seed thus sown may bring forth an abundant harvest.

I would premise that I do not advocate photography in microscopic science as a rival that will supersede the draughtsman, except in certain cases ; and although it may in very many instances do so, it will most assuredly make much more work than it takes away from those who follow the occupation of a microscopic artist.

When the object to be delineated is flat and moderately thin, as compared with the necessary power in use, a very excellent picture may be produced without any aid from the limner; but where the object is not so formed—although when under microscopic examination the mind can readily acquire a correct knowledge of the form by focussing up and down—it is evident that from the very construction of a good objective a picture can only be obtained in one plane at a time, and it will then be necessary to take several pictures in different planes, and call in the artist’s aid to unite the productions. The immense amount of time and labour that can be thus saved in delineating subjects of an elaborate character can only be appreciated by those who have attempted the production of objects of this class.

It is scarcely necessary to enter into a preliminary explanation of the photographic phenomena, as it is of very little use for an entire novice in the practice of this art to commence upon microscopic subjects; I shall, therefore, presume that I am addressing those who understand the general principles of photography, and shall therefore commence with

The Arrangement of the Apparatus.—Place the microscope with the body in a horizontal position, and screw on the objective to be used, and fix the object in its proper position on the object-plate of the stage by pressing down the sliding spring-piece. Turn the mirror aside or remove it altogether, and having taken out the eyepiece, insert into the body a tube of brown paper lined with black velvet, in order to prevent the slightest reflection from the sides, which would infallibly spoil every picture if allowed to operate. The lens should then be removed from an ordinary photographic camera, and the latter elevated so as to bring its centre in an exact line with the axis of the microscope body, which must have its eyepiece-end inserted in the place left vacant by the removal of the camera lens, and that portion of the opening not filled up by the body may be rendered impervious to light by a piece of black cloth, velvet, or other similar material.

The lighted lamp must next be brought, so that the centre of the flame is in the axis of the instrument, and its distance must depend upon the focus of the lens used to concentrate the light, for which purpose an ordinary convex lens of $212$ to 3 inches diameter, with its flat side towards the lamp, is perhaps as useful as any, provided a second plano-convex lens of that focus is interposed near the object to concentrate the light still more strongly. It is not necessary, or even desirable, that an image should be formed of the source of light, and consequently the spherical aberration in such an arrangement as recommended is not detrimental, and may be advantageous.

The ground glass screen to receive the image being in its proper place in the camera, the object may be brought to a correct focus in the usual way with the coarse and fine adjustment, and this cannot be done too accurately ; in fact, for delicate objects, a means of magnifying the image is absolutely requisite, and for this purpose a positive eyepiece, placed in contact with the ground glass, is perhaps best.

Most achromatic objectives of the best construction are slightly over-corrected (as it is termed) for colour, in order to compensate for a small amount of under-correction in the eyepiece, that is to say the violet and blue rays of the spectrum are therefore projected beyond the red ones.

As it is ascertained that most of the photogenic or actinic rays are located in the violet end of the spectrum, it follows that with such a lens as is used for the microscope, the chemical focus will be somewhat more distant from the object than the visual focus, and it therefore becomes necessary to make some allowance for this difference.

This may be done in two ways, either by placing the sensitive plate somewhat farther off than the ground glass on which the image is received, or by altering the focus by the fine adjustment; the latter being the plan I prefer, as I find it much more accurate.

The amount of difference between the foci probably varies in every objective, even apparently of the same make, and can only be ascertained by direct experiment, but the following may be some guide to those who wish to experiment upon the subject.

An inch-and-a-half objective of Smith and Beck’s make required to be withdrawn from the object after the correct visual focus is ascertained 1-50th of an inch, or two turns of their fine adjustment.

A two-thirds of an inch object glass of same make wants a withdrawal of 1-200th of an inch, or a turn of the fine adjustment, and

A 4-10ths of an inch, about 2 divisions, or 1-1000th of an inch farther off. With the 1-4th, and higher powers, the difference between the foci is so minute that it is practically unimportant. The above differences are those actually existing in my own objectives, but, as before intimated, it does not follow that they will be correct for others even of the same makers.

Having arranged the apparatus, focussed, and made the requisite adjustment for chemical focus, the ground glass may be removed, and the sensitive plate placed in its stead.

As in all other photographic processes, the time of exposure must be varied according to the power in use, the nature of the object to be taken, and the amount of illumination, to which must be added in the present instance the medium in which the object is mounted, but from 1 to 10 minutes’ exposure is generally requisite. An explanation of the last named disturbing cause may probably be found in the beautiful discovery of Professor Stokes of the property possessed by certain transparent media of arresting the chemical rays.

Any account of the preparation of the collodion, &c. &c. would be more fitted for a work on photography, and would render the present paper much too lengthy: moreover there is an abundance of information on photographic manipulatory details readily accessible in numerous publications, such as Mr. Robert Hunt’s Manual, Mr. Bingham’s, Mr. Archer’s, Mr. Horne’s, Mr. Hennah’s, &c. &c. There are, however, one or two points which it is as well to allude to. If the film of a collodion picture be examined by the microscope, some specimens will present an appearance very much resembling condensed cellular tissue, such as that seen in the cuticle of leaves, being apparently made up of flattened irregular hexagonal cells ; while others seem to consist of an entirely structureless amorphous mass ; the latter sort of collodion is most suitable for microscopic purposes.

The final fixation of the picture by removal of the iodide of silver has a singular influence upon the result according to the method employed, and advantage may be taken of this in order to improve the effect according as it is desired to produce glass positives or negatives ; for though all collodion pictures partake of both characters, one of the two should always be predominant.

Of course a negative is most useful, because the drawings can be multiplied upon paper almost ad infinitum, but for certain objects the amount of detail when very delicate is inconceivably better shown upon glass than upon paper. If then a negative picture be desired, it is best to develope with the pyrogallic acid solution, and fix with a solution of hyposulphite of soda; but if, on the contrary, a positive picture is the desideratum, the effect will be infinitely bettei-by fixing with a bath of the following, viz.:—

The cyanide to be dissolved in the water, and the crystals of nitrate of silver added, which immediately cause a curdy precipitate, but this is quickly redissolved, and the whole becomes quite translucent.

By this method of fixing, the lohites are very much purer and brighter than when the hyposulphite is used, but the pictures do not answer so well for printing from. A still further intensity of the whites may be produced by developing the picture with a solution of the proto-sulphate of iron, instead of the pyrogallic acid, and afterwards fixing with the cyanide solution; there are, however, certain difficulties of manipulation to overcome. The solution is made as follows:—

This is best used by placing in a yZass bath and totally immersing the plate, which should be withdrawn the moment the picture is perfectly developed, which will be in from 15 to 60 seconds, and it ought to be instantly plunged into a bath of plain water sufficiently copious to dilute the adherent moisture very considerably. The object of the bath being of glass, is in order to see the development of the picture, as every second it remains after it is fully produced, is to the detriment thereof, by causing a sort of fogginess to appear all over it.

When developed with the protosulpbate of iron, the pictures may be exposed to direct day-light before the final fixing, without injury, in fact with positive benefit according to Mr. Martin.

The causes most frequently operating to prevent the success of the process are, first, want of attention to the proper illumination ; it is to this point more than any other that the utmost attention should be paid, and I feel confident that by well concerted measures to attain this requisite, we shall eventually be able to obtain pictures in a tithe of the time now necessary ; in the second place failures more often occur from over exposure.than from being too short a time; thirdly, want of allowance for difference of visual and chemical foci.

In conclusion, I would observe that some experiments upon the different light-producing substances would in all probability well repay the trouble of testing their capabilities, as from certain hints thrown out by Professor Stokes, there appears to be a very considerable difference in the amount of actinic rays emitted by differing combustibles, and it seems not improbable that a well contrived spirit lamp may be found highly advantageous to use while taking the impression, although its lightgiving properties are so defective. I hope shortly to be able to resume this subject.