The photographic news

• Bandzählung: 29.1885
• Erscheinungsdatum: 1885
• Sprache: Englisch
• Signatur: F 135
• Vorlage: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: SLUB Dresden
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394 THE PHOTOGRAPHIC NEWS. [June 19, 18? 5. carrying an achromatic condenser, polariscope, &c., are almost indispensable. The objectives should be the very best, specially corrected for photography in all powers under one-fifth of an inch, and of moderate angular aperture. Perfect corrections of chromatic and spherical aberrations, with the greatest pene trating and defining powers, are essential features in objectives designed for photography. Without making any invidious com parisons, I may remark that all these features are embraced in the object glasses of R. and J. Beck, and can heartily recommend them for this purpose. A binocular microscope can be used quite as well as a monocular by simply shifting the prism ; but if an instrument is to be selected purposely for photographing, the latter is recommended in preference to the former. In either case the tube is to be lined with black velvet or flock paper, otherwise there will be a reflection from its inner surface, which will cause a ghost in the negative. An accurately centred double or triple nose-piece, carrying as many objectives of different powers, will also be found an immense convenience, saving much time and labour in choosing the proper object glass for any particular object. I use a triple nose-piece, upon which are always screwed a two-thirds, a four-tenths, and a one-fifth inch objective, being the powers most generally used. If the one-fifth is to be employed in making an exposure, the object is readily brought into the centre of the field under the two-thirds, by a simple rotation of the nose-piece ; the one-fifth is now brought into position, when the object will be found to occupy the centre of its field of view. Much valuable time, which would otherwise be expended in finding a minute object under so con siderable a power, is thus saved. Most chemical crystals, being colourless and nearly invisible under ordinary illumination, require to be examined and photo graphed by polarized light. For this purpose, the polariscope prisms should be of large size, and both mounted in revolving settings. The lower prism should also fit into a sub-stage capa ble of carrying an achromatic condenser above, in order to properly illuminate the field with any objective above the two- thirds inch. The achromatic condenser itself should consist of, at least, two combinations of different powers, and of wide angular apertures. It is always necessary to employ a condenser of greater power than the objective. If a one-fifth is used, the illumination should be by a four-tenths to get the best results, whilst a one-tenth should be illuminated by a one-fifth. For lower powers, the ordinary bull’s-eye condenser, in connection with a diaphragm of varying apertures, will be found alls-uffi- cient. For the great majority of workers the following powers of object glasses are recommended: 2 inch or 1} inch, 3 inch, 1 inch, ! inch, andinch, the latter an immersion, capable of being used with glycerine, since water evaporates too rapidly for practical use. With these powers, and a camera having thirty inches extension of bellows, a range of powers from about ten to six hundred diameters may be obtained ; whilst the addition of an achromatic amplifier will increase the power of the Ath to more than one thousand diameters without per ceptible loss of light or definition. All of the foregoing features are combined in the microscope I am using for this work, and, in enumerating them, I have but described that very perfect and complete instrument. We pass now to the second part of our outfit—the camera. Probably the most convenient and generally useful form is one with bellows extending some thirty inches, and capable of carrying both quarter and half-sized plates, horizontally or vertically. My box is a square one, carrying plates from the 4-4 size (6} by 8+) down to 3| by 41. The bellows are in three divisions, extending fully four feet, giving altogether nearly six feet from the microscope stage to the focussing screen when fully drawn out. It is very carefully made of hard wood, and the framework carrying the bellows moves with the utmost smooth ness upon its V -shaped ways. A short cone front receives the microscope tube, and all extraneous light is shut out by a wrapping of black velveteen around the opening, secured by a rubber band. The focussing screen (which is only used for arranging the object in the field or plate, and is entirely re movable) has a space of the precise size and shape of a lantern slide pencilled in the centre, as a guide to the making of - plate negatives. An object filling this space on the ground glass will necessarily occupy the same position on the quarter-plate when the holder containing the latter is made to take the place of the focussing screen. Tne arranging of the object in the centre of the field (under a low power) and the coarse adjustment of focus are done with the bellows tightly closed, which brings the focussing screen so near to the microscope that, whilst the operator sees the object thereon, his hand can readily reach the milled head controlling the stage and other movements. But when the bellows are extended to the length affording the desired magnification, it will be found that the object, whilst retaining its central position, has lost in sharpness, necessitating a final and careful focussing, which is no longer possible in the same manner as before, since the ground glass and microscope are so widely separated that the one cannot be reached whilst looking into the other. Some special device becomes necessary in this emergency, and many have been made, mostly complicated and costly. The method I have adopted is the old and simple one of a fine cord passing around the periphery of the milled head controlling the fine adjustment, in a groove cut for that purpose ; thence through a series of screw-eyes to the roar of the framework, carrying the bellows extension, where it is kept taut by a couple of small leaden weights. The slightest tension upon this cord causes a corresponding movement of the fine adjustment, and nothing can exceed the delicacy of its working or its freedom from derange ment. 2k fine fishing line maker an admirable cord for this purpose. As stated, the ground glass is used only for centering the object in the field, and for coarse adjustment of focus. It is im possible to grind it finely enough to accurately focus any delicate tissue, with high or even moderate powers. Many suggestions have been made and devices used, but all I have tried proved unsatisfactory, until the following appeared in an English journal: — “An evenly-coated gelatine plate is to be expostdtoa flash of white light, developed to a very slight intensity—a mere smoki ness—fixed and washed as usual, then bleached with mercury, washed and dried. The result is the most perfect focussing surface imaginable. To use : Remove the focussing screen from the camera, and replace it with the plate holder, from which the slide and back have been removed. The bleached gelatine plate (which, of course, must be of proper size to fit holder) is to he placed therein with the film side towards the microscope. Now place a focussing glass against the back of the plate, apply the eye to it, and adjust the focus by a delicate pull upon the cord. Nothing can be more satisfactory. When the proper adjustment is determined upon, secure it from possible change during ex posure, by winding the cord once or twice over the screw eye at the rear of the camera frame.” The plate-holder (single) opens at the back to receive the plate, and being square like the camera box, admits of the negative being taken vertically or horizontally, as may be desired, a matter of no small importance in many cases. This feature in connec tion with the revolution of the stage enables one to place any object upon the plate in the best position for printing. The full size of the plate carried by my holders is 6} by 81, but by the employment of removable rabbett kits, plates 5] by 7, 4} by 5} and 3| by 4 J can be used. It is desirable to have two or three additional holders as time-savers. For our work to possess any real uniformity or value, it is absolutely necessary that the magnifying power used with each exposure should be ascertained and noted. To make the neces sary measurements each time would be a waste of that precious commodity, to avoid which I have adopted the following device. The bed or framework upon which the bellows extends is divided into spaces of one inch, and the same plainly marked thereon. Suppose this extension is thirty inches, and that by the extension of the stage micrometer we find the 1} inch ob jective to magnify twenty diameters with the bellows closed, and fifty, fully extended. This gives the value of one diameter to each inch of bellows drawn out, and if the magnified object reaches the desired dimensions, at, say, twenty inches extension, we at once know the power to be forty diameters. Suppose, again, a power of one-fifth is being used, magnifying 1'25 dia meters with bellows closed, and 275 with same extended thirty inches, this gives a value of five diameters to each inch, and if the desired size is reached at twenty-five inches of extension, we have a power employed of 250 diameters. By making these two measurements (closed and open), for each objective habitu ally used, and recording the fame in our note-book, an accurate and readily adjustable table of measurements is ready for all future work. We now pass (thirdly) to the important subject of illumina tion, in many respects the most important in photo-micrographic work. It goes without saying that to the favoured few whose time and means permit the harnessing of the sun s rays in their