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The photographic news
- Bandzählung
- 27.1883
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1276, February 16, 1883
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- Titelblatt Titelblatt I
- Register Index III
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109 ditch, or seaside pool, will afford living examples for the micro scopist. I diverge for a moment from the immediate subject of my paper to describe these peculiar organisms on account of the beautiful skeletons they contain, which form favourite subjects for the photo-micrographer, the valves presenting a flat plane to the focussing-screen, and exhibiting wonderful variety in the sculp tured markings on their faces. As a rule, they consist of two plates of siliceous material covered with delicate patterns, the plates being held together by a band or hoop of similar material, the whole forming a sort of flat case—in some specimens like a round shallow snuff-box, and containing protoplasm within. For microscopic purposes the specimens are boiled in acid, and the clean shells mounted dry or in balsam. These examples of the German photographers, however, do not in any way exceed in beauty the work privately published by my late lamented friend, Dr. Redmayne, of Bolton, whose book of diatom photographs is also here for inspection. Dr. Woodward, of Washington, has kindly sent me for the pur pose of this paper a print of the diatom, Surirella Gemma, as an example of high magnification and the resolution of difficult markings. Few microscopists comparatively succeed with their own instruments in resolving the lines of dots thereon, even with high powers, their resolution being much more difficult than that of the common test object, Pleurosigma Angulatum, to which I have before referred. This photograph was taken with a lens of *s focus, made by Powell and Lealand, and the magnification is 2,800 diameters, the dots, so plainly seen, each measuring about 37800 of an inch, but I cannot say whether the negative Was afterwards enlarged. Those interested in this subject may find magnificent specimens of Dr. Woodward’s work in the libraries of the Royal Micro scopical Society at King’s College, and of the Microscopical Society of Liverpool. Apparatus.—The apparatus employed need not necessarily be expensive. Any small microscope, with fine and coarse adjust ments for focussing, and an ordinary quarter-plate camera arranged on a short base-board, will serve for a beginner ; and even the camera and microscope stand may be dispensed with by the nse of a dark box, as described in the English Mechanic for February the 2nd, 1883. It is preferable, however, to have a base-board four feet long and eight inches wide, provided with a ledge of wood half-an-inch square on each side, between which a block or carriage may slide, and upon which a bellows camera can be fixed at such an altitude that the flange for the lens will admit the eye-piece end of the microscope, which must be placed in the horizontal position. To allow of a wide field it will be well to have the body or tube cut off about an inch from the nose, and a screw-collar arranged to allow of replacement of the tube when it is desired to use the instrument in the ordinary way, or to photograph with the addition of the eye-piece. A short cardboard tube fitting into the lens flange of the camera will allow of a taper velvet collar two or three inches long being glued thereto, which will serve to keep out the light, and yet allow free movement of the object-glass for focussing. The bellows should possess the greatest latitude of expansion, allowing a variation of length of focus from ten inches to thirty- six inches. The interior of the microscope body should be lined with black velvet to prevent flare on the plate from reflec tion. When the camera is extended to the full limit, a piece of looking-glass held in the left hand at a suitable angle opposite the ground glass screen will enable the operator with his right hand to focus roughly with the coarse adjustment screw. For accurate focussing a rod passes under the camera, having at one end a knob as a handle, and at the other extremity a pulley one inch in diameter provided with a V groove, in which runs an end less cord working the button of the fine adjustment also grooved. The rod and pulley should work smoothly to avoid uneven strain on the arm carrying the objective, and the fine adjustment should run sweetly, and answer immediately to the least turn of the rod. A mechanical stage with rectilinear motions for carrying the ob ject to be photographed is very convenient, but not essential. The stage plate should, however, be furnished with levelling screws at the corners for bringing all parts of the picture into a flat plane. Both of these appliances are to be seen on the instrument before yon. On the shaft of the fine adjustment screw a short split brass tube half-an-inch long is made to slide stiffly, to which a stout wire pointer three or four inches long is soldered. A semicircle of cardboard, having its centre coincident with the axis of the fine adjustment screw, is placed behind the pointer, and marked in circumference with degrees. The split tube allows of the entire revolution of the adjustment screw, whilst the pointer acts as n index through 90“ of arc, and records with exactness any slight alteration in focus. The necessity for this will be hereafter ex plained. As correct focussing is a most essential point in photo-micro graphy, the finest ground glass is inadequate for viewing the image for final adjustment. The screen is therefore removed, and a lens of short focus mounted in the middle of a light lath, two inches wide and fourteen inches long, is so adjusted in a short tube that its focus is conincident with the plane of the sensitive film when the lath is held against the back of the camera. This operates as an eye-piece, and the adjusting rod is turned till the picture appears in sharp outline. At the side of the base-board a scale of inches is marked, measuring from the object carrier for recording the length of focus employed. Scales of diameter under the respective object glasses can also be added, showing at a glance the magnification obtained. Thus at thirty-six inches, it is definitely seen on this instrument that the two-inch objective gives twenty-one diameters. The chief difficulties to be surmounted by the novice occur under the following heads :—1. Non-coincidence of actinic and visual foci of lenses. 2. Even illumination. 31 Exposure. 4. Selection of suitable objects for photography. {To be continued.') Uorrespondence. BROMO-IODIDE IN EMULSION. Sir,—I have just seen Mr. Penny’s communication of the 26th January, regarding the introduction of iodide into gelatine emulsion. I wish to offer one or two remarks re garding the question. In 1877-78, when Mr. Penny’s article in the Year-Book was written, gelatine emulsions had not got out of their infancy. They could toddle, and not run. The rapid processes by aid of digestion, boiling, and ammonia had yet to be developed ; and it was for the slow process that Mr. Penny recommended the addition of iodide, the advantages of the addition are more pronounced and more numerous in the rapid processes than in the last-named one, in which it has about the same value as in a collodion emulsion, as Mr. Carey Lea has shown. From the time of the article in the Year-Book, 12th January, 1878, till the 26th January, 1883, Mr. Penny has, I think, been silent regarding it. In the interval 1 have borne the brunt of the battle of “iodide” versus “no iodide” in rapid emulsions; and now that the value of iodide “is being recognised,” he comes forward and wishes to father the child which I have endeavouied to bring up so properly. His help when “ the addition was not duly appreciated ” would have been most welcome.—Yours faithfully, W. de W. Abney. Proceedings of Socketies. Photographic Society of Great Britain. The ordinary monthly meeting of the above Society was held at 5a, Pall Mall East, on Tuesday evening, the 13th inst., James GrATeIIER, Esq., F.R.S. (president) in the chair. The minutes of the previous meeting having been read and confirmed, Messrs. Leslie, Robertson, Edwin Bell, and Alfred Isenberg were duly elected members of the Society. The Secretary’s and Treasurer’s reports for the past year were read and adopted. The Chairman then proposed a hearty vote of thanks to Mr. Bird (the treasurer) for his untiring exertions for the good of the Society. Mr. John Spiller, in seconding the motion, said that, speaking from an experience of five years, he could heartily appreciate the duties which Mr. Bird had performed, and he congratulated that gentleman on being able to show so satisfactory a balance- sheet. He thought it was a matter for congratulation that the
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