The photographic news

• Bandzählung: 35.1891
• Erscheinungsdatum: 1891
• Sprache: Englisch
• Signatur: F 135
• Vorlage: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: SLUB Dresden
• Rechtehinweis: Public Domain Mark 1.0
• URN: urn:nbn:de:bsz:14-db-id1780948042-189100009
• PURL: http://digital.slub-dresden.de/id1780948042-18910000
• OAI: oai:de:slub-dresden:db:id-1780948042-18910000
• Sammlungen: LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
• Sammlungen: Fotografie
• Strukturtyp: Band

• Digitalisat: SLUB Dresden
• Strukturtyp: Ausgabe

February 6, 1891.] THE PHOTOGRAPHIC NEWS. 103 image on to the screen at one foot from the microscope, and, with a pair of compasses, measure off its amplification with that power on an ordinary rule, and thus arrive at the number of diameters it is magnified. I then do the same thing at two feet, three feet, and so on. I thus register the magnifying power of that object glass at these several distances. I adopt the same procedure with all the objectives I use, and thus form a table of measurements, to which I can readily refer in all cases. This apparatus of mine is comparatively simple and inex pensive, a matter to be considered in the absence of any regular department in the State set apart for our assistance in these matters. There is another advantage in this arrangement, and that is I can do all my work in one room, as the velvet curtain shuts in the lamp, and 1 can develop my negatives in the same room. But I must not take up more time in explaining this appa ratus ; a lantern slide to be passed through this even ing will give a clear explanation of its construction. It may be necessary to say that all generally required is an ordinary bellows-camera, or, even better, a sliding camera, placed on a firm base-board of well-seasoned mahogany or teak, about five feet long and one and a-half inch thick, and sufficiently wider than the camera to admit of a wooden ledge on each side being fixed, to form a broad groove in which the camera can slide backwards or forwards as required, with a thumb-screw through its tail-board to clamp it at any desired distance. This base-board must be supported in such a manner that any vibrations from passing traffic, or persons moving about the house, may not be communicated to the apparatus. I accomplish this by standing it on a fairly good sized bundle of journals placed at each end, and I find this stops the vibrations so far that very little evidence of a tremor is manifest in photographs I have taken up to 200 diameters. I now come to that division of my subject whicii treats of the microscope ; it is essential that this should be a steady, solid monocular, having a good, fine adjustment, for this is very necessary in getting absolutely sharp definition. Another indispensable adjunct is a substage condenser, fitted with a revolving diaphragm pierced with holes of varying diameters. In my rotating diaphragm attached to the condenser the apertures vary from 1-16th of an inch, the smallest, to -inch, the largest. The smallest is of no use in photographing, but it serves a very important office in enabling me to centre the light. Presuming that your camera and microscope are axially centred with each other and the lamp, it does not follow that the beam traversing these must be central, and though a great deal of this excentricity may be overcome by moving the lamp, it is not entirely overcome, but if the substage condenser has adjusting screws—which it ought to have— perfect centricity can be attained in this way : put in a very low power, and, having turned the diaphragm to the smallest aperture, project its image in the middle of the focussing screen, and get its borders quite sharp ; now rack the condenser back, and the field will be found evenly illuminated. Should the low power be now changed for one of a higher grade, a further slight adjust ment may be required, but, having already a rough approxi mation, this may be readily obtained. The centreing of the light is a most important proceeding, for, unless this is attended to, one side of the object will have a dark and fuzzy border, while the opposite side will be light and •coloured, and what is thus easily perceptible in the borders extends throughout the whole object, and detracts from the general sharpness of the photograph ; therefore, too much attention cannot be devoted to this point. The tube of the microscope should be lined with a dead-black paper, or a flare-spot from internal reflection will make itself obnoxiously evident in your negative. Some recom mend black velvet as a lining ; this answers very well, but, being thicker than paper, it helps to diminish the already- constricted field. The tube in my microscope is trumpet shaped, which allows the rays from the object to diverge immediately on passing through the lens, thus removing the constriction, and neutralising any tendency to a flare spot. I might also say that I never use the eye-piece of the microscope, as I find for my purpose I can do better without it. While we are discussing the microscopic division of this subject, I would especially recommend that the lamp should be kept burning for some quarter of an hour or so before attempting to photograph, to allow of all the con stituent parts of the apparatus to warm up and expand. The necessity for this is easily demonstrated by focussing your object as sharp as possible, and let everything remain undisturbed for about ten minutes. When looking at your object again you will find all sharpness has dis appeared, and nothing but a blurred, indistinct mass is evident: but let expansion once take place, and no alteration of focus will occur the rest of the evening. I may add that in working with powers less than one inch focus, a great advantage will be derived from interposing a piece of ground glass between the bull’s-eye lens of the lamp and the sub-stage condenser, its ground surface being placed next to the object. This, while it makes the light a trifle less powerful, and slightly prolongs the exposure, gives a soft, diffused light by its acting as a radiant. The plan I have always adopted for obtaining a sharp image on the sensitive plate is one that I may recommend straight away, and thus save you the trouble of trying the various expedients suggested by many practitioners, but without satisfactory results in my hands. I say, remove the ordinary ground glass focussing screen from your camera, and replace it by a plain glass having some fine lines drawn across it with a writing diamond, these lines being placed next the objective. Now set your focussing glass till these lines are sharply defined. These lines will closely approximate to the plane of your emulsion on the sensitive plate, and, therefore, if you get the details of your object into focus with the lines on the focussing screen, the probability wiil be in favour of a sharp image on the negative. The lamp I employ is an ordinary microscope lamp burning Strange’s crystal oil, in which some camphor is dissolved to increase the whiteness of the light. A plano convex bull’s-eye lens is attached to it. Much discussion has taken place with reference to the position of this lens, some saying that the plane side should be next to the light, others advocating the reverse. I have tried very carefully the results to be obtained in both positions, and I cannot detect any difference in them; but, if anything, I prefer the flat side away from the lamp, and next to the condenser. I don’t pretend to any knowledge of the laws of optics, and I only speak from my experience. Having got these three constituent elements—camera, microscope, and lamp—as accurately centred as it is possible, and the apparatus warmed up, we may proceed I to operate; and here the question of exposures crops up,