PhotoHistory

THE BRITISH JOURNAL OF PHOTOGRAPHY. No. 1392. VOL. XXXIV. JANUARY 7, 1887.

PHOTO-MICROGRAPHY.

By W. H. WALMSLEY,

[Abstract of Paper read before the Photographic Society of Philadelphia.]

Any stand, monocular or binocular, the body of which can be inclined to a horizontal position, may be employed. In using a binocular stand it is merely necessary to withdraw the prism sufficiently to allow all the light to pass up the straight tube, and thus convert the instrument into a monocular. The eye-piece may be removed or not as the operator desires, the former plan being the one generally adopted and producing, so far as my own experience goes, the better results. If the eye-piece is not used, the tube must be lined with some non-reflecting material. A mechanical stage is a great convenience and time saver in centering an object upon the ground-glass, but is by no means a necessity; it should, however, revolve in the optical axis of the instrument if possible. A double or triple nose-piece carrying as many object glasses of different foci is also very desirable, especially when high powers are to be used, as an object can be quickly brought into its proper position in the field of view under a low power, when by revolving the nose-piece a higher one takes its place, and if the two are properly centered, the object will be found to occupy the same position in the new field of view.

The placing of the object on the stage of the microscope, and arranging it in such a position as to cast its illuminated image upon the center of the ground-glass focusing screen, are all to be done with camera closed up short, in which position the eye can readily see the image, whilst the hands reach the milled heads controlling the stage and focussing adjustments. But when the bellows are extended a foot or more this becomes impossible, and resort to some other method of varying the focus is necessary. The plan I adopted at first, and have ever since followed, was the old but effective one of a strong, line cord placed in a groove turned in the periphery of the milled head controlling the fine adjustment of the microscope, and passing through hook eyes on either side of the camera to its rear, where it is held taut by a couple of small leaden weights. A very slight pull on either of these serves to change the focus in the most delicate manner, and this can be done whilst the eye is fixed upon the image on the ground-glass.

I have used the term ground-glass in connection with the fine focusing adjustment, but merely as a figure of speech. In reality ground-glass cannot be used for this purpose at all, as the very finest that could possibly be made is entirely too coarse to define sharply any delicate microscopic object. Its usefulness is confined to securing a full and even illumination of the field, and the arranging of the object in its proper position therein. These two essentials arrived at we must abandon the ground surface in favor of something better adapted to giving a sharply defined image of the magnified object. Many plans for effecting this have been published, but I shall at present notice only the two which I have used most successfully for some years. The first, which is useful for hurried work, consists in attaching to the center of the ground surface of the focusing screen, by means of Canada balsam, a disc of thin microscopic covering glass about an inch in diameter. The balsam fills up the minute inequalities in the surface of the glass made by the grinding process, and renders the circle perfectly transparent. Now, if a Ramsden positive eye-piece, or any ordinary focusing glass adjusted to the thickness of the screen, be applied to the outer side thereof, the illuminated image will be seen clearly defined in this transparent circle with the most beautiful distinctness, when the proper focus is made with the adjusting cord.

This method, however, only allows that portion of the object which is in the center of the field to be seen and focussed upon, whereas it is frequently desirable to go all over the same to arrive at the best average adjustment. For this purpose I use the following plan, which is so perfect in its results that I can conceive of nothing better: A sensitized gelatine plate, evenly coated, is exposed to a flash of light, and placed in the developing bath a short time, until it becomes I somewhat discolored, or clouded over, but not long enough for it to acquire any destiny. It is then washed and fixed as usual; after fixing and a prolonged washing, it is bleached with bi-chloride of mercury as for intensification, and again washed and dried, when it will be found to possess the finest surface for delicate focusing imaginable. And now comes in the use for the single plate holder, to which I have before referred.

Slide and back both being removed, the holder is reduced to a mere framework, which is placed in position on the camera, the ground-glass screen having been lowered out of the way. The bleached gelatine plate is now placed in the holder (film side toward the image), where it occupies exactly the place that will subsequently be filled by the sensitized plate. The positive eye-piece, or focusing glass, being now placed against the outer surface of the plate, and the eye applied thereto, perfect definition may he effected by means of the focusing cords, the proper position of the image upon the plate having been previously secured through the medium of the ground-glass.

A few words as to the objectives may not be amiss. For powers including one-quarter of an inch and higher, no special corrections are necessary. I have had most excellent results with the higher powers of very many of the best makers, both American and foreign, and even the cheap French triplet quarters have produced very good work without further correction. But for all lower powers a special correction, to render the actinic and visual rays coincident, becomes necessary for perfect work.

The proper illumination of the object is of the first importance, since, if we fail in this, all our other efforts are of but little account. Various sources of light are at our command. The direct rays of the sun, diffused daylight, the electric light in various forms, magnesium ribbon, or the oxy-hydrogen jet, can all be used most successfully by those able to command time and apparatus. But my remarks at this time being intended mainly for the benefit of those who have only their evenings at their disposal, I shall confine myself solely to the consideration of that illuminator procurable by everyone — a good lamp burning coal oil. Any form capable of being placed at a proper height to suit the stage of the microscope may be used, though some are undoubtedly better than others. The one I have adopted, and find eminently useful, consists of a broad, flat reservoir, with large capacity, carried at any desired height upon an ordinary retort stand, and using a duplex burner, with the flames placed at an angle to the plane of the stage, in order to avoid the dark spot in center of field which always results if the flames present their edges to the object slide. The stage of the microscope should be provided with a set of diaphragms of various sizes, the arranging of the object being done with a large opening, whilst the exposure is made with the smallest which will properly and evenly illuminate the field of view. If desired, a condenser can he used to concentrate the light upon the object; but for a long time I have entirely discarded its use with all powers lower than one-fifth of an inch, using the direct rays proceeding from the lamp alone. This gives a more evenly lighted field, and although the length of exposure is somewhat increased, the resulting negative is better in every way than one made in a shorter time by a strongly concentrated light.

The difference in fact is very marked, and I would urge upon everyone using lamplight to dispense with all condensers with moderate and low powers. With high powers, however, a condenser becomes necessary, and an achromatic combination of wide aperture, or one of the Abbe form, may be used with the best results. Some operators use blue or ground-glass for toning and diffusing the light. In short, the subject of illumination is an exhaustless one, and I shall not attempt to pursue it further at present. Neither can I give any certain rules as to time of exposures. The length of these vary greatly with the nature and structure of the subject, so that every one must learn to determine this for himself, as in ordinary photographic work. Using a given objective, and with the same plate and illumination, I have had exposures vary from ten seconds to five minutes, regulated entirely by the difference in the color and structure of the subject to be photographed.

For all delicate structures, especially finely marked diatoms, I find it much better to develop for detail only, resorting to after intensification to obtain necessary density. By this means none of the finer lines or markings are obscured by too heavy a deposit, whilst the negative is quite as permanent as those not intensified, if it be first bleached in a solution of bi-chloride of mercury, and blackened by immersion in cyanide of silver, or a ten percent solution of sulphite of soda. Ferrous oxalate or pyro development may be used with equally good results. At first I favored the former, but of late years have, in common with most others, grown more and more accustomed to pyro, which, with carbonate of soda, has given me results uniform in their success, and of a quality leaving little or nothing to he desired. I use no restrainer excepting the sulphite of soda in solution with the pyro, the resulting negatives being uniformly of a pleasant gray tone, entirely free from the yellow stains so often associated with the use of pyrogallic acid.

For making prints, bromide paper is especially recommended. It can be used in the camera for enlarging or reducing, or be printed by contact, by day or lamp light, as desired. It renders all delicate details better than albumen, the finished prints need not he mounted, for book illustrations they are perfect, whilst, finally, I think there can be no reasonable doubt of their absolute permanence. If a glossy surface is desired, it may be obtained by squeegeeing the print upon a sheet of polished hard rubber, from which it will peel off with a glazed surface when dry.

Since the making of transparencies from microscopic negatives differs in no wise from those of other subjects, I shall not detain you in alluding to them further than to repeat that reduction in the camera gives more satisfactory results than contact printing, and that this is the plan I almost invariably pursue, even if the negative be upon a quarter-sized plate.

It adds very greatly to the scientific value of a photograph of any microscopic object, if the exact magnification be measured and stated thereon. To do this separately with each exposure would entail an expenditure of time and labor that few would care to assume, but if each objective be carefully measured with the camera closed, and also opened to the full extent of the bellows, and notes thereof preserved, it will he an easy matter to calculate the exact amplification used with each exposure. This measurement is very easily made. A stage micrometer ruled in one-hundredths and one-thousandths of an inch is placed upon the stage of the microscope, and the image thereof focussed upon the ground-glass, where the value of each division is readily determined. I would advise everyone turning his attention to this branch of photography to make such a series of measurements at the outset.

As previously stated, any microscope with a joint for inclination of the body to a horizontal position may be employed in photography, but those having a short tube of large diameter will do better work than those with long ones. It is highly desirable that the rays of light, after passing through the objective, should be allowed to diverge as speedily as possible. Where they are confined to the limits of a long tube of small diameter, it will be found that only a small circle of light will be thrown on the ground-glass when the camera is closed up short, and that it must be lengthened to a considerable extent before the entire plate is illuminated. In order to overcome this objection, I devised, some years since, a stand from which the compound body may be entirely removed, and replaced by a cone-shaped tube which flares away from the back of the object glass, allowing the image-bearing rays to diverge at once after leaving the lens.