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
• Rechtehinweis: Public Domain Mark 1.0
• URN: urn:nbn:de:bsz:14-db-id1780948042-188500006
• PURL: http://digital.slub-dresden.de/id1780948042-18850000
• OAI: oai:de:slub-dresden:db:id-1780948042-18850000
• Sammlungen: LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
• Sammlungen: Fotografie
• Bemerkung: Seite I-II fehlen in der Vorlage. Paginierfehler: Seite 160 als Seite 144 gezählt.
• Strukturtyp: Band

• Digitalisat: SLUB Dresden
• Strukturtyp: Ausgabe

[August 21, 1885. THE PHOTOGRAPHIC NEWS. 540 prisms together, two of a light glass and one of a denser, and we get a combination, in which, although the main beam will pass straight on to the screen, yet the presence of dispersion is also shown by the formation of a spectrum. This is an example of what is called a direct vision prism. The spectrum is given by the differences of the refractive indices for each ray in two kinds of glass. For some purposes this kind of compound prism is very useful, and particularly for lecture experiments ; but, as a rule, for photographic purposes I should not recom mend it, on account of the internal reflections which take place between the different surfaces of the glass, though they are cemented together. You must recollect, wherever there is a difference in density between two media—in other words, a difference in the refractive indices—there is always a certain amount of reflection, and those reflections, being white light, are rather apt to fog the plate, and give you false notions of what you get in the photograph. We come now to a much more important point with regard to the spectrum, and that is, what is the best material to use. In those prisms which I have already shown you, the material was glass. Now glass is, comparatively speaking, a mixture of materials, and has no definite chemical formula; but when we come to a material which has some definite chemical formula, we find that, as a rule, it has certain properties which are in valuable in certain forms of spectroscopy, more particularly when the photographic plate has to be brought into requisition. Quartz is an example of this ; it is a definite compound of sili con and oxygen, and we find that it has certain definite advant ages which are not to be found in glass prisms. The dispersion is not quite so great as it is with glass, but, on the other hand, it lets through rays which are cut off completely by glass, as I hope to show you on the screen. This quartz prism has very well-worked faces, and we will send a beam of light through it, and then proceed to investigate its behaviour. [Spectrum thrown on screen ] I may further say, in reference to this, that the condenser in that lamp is quartz, the lens is quartz, and the prism itself is quartz, so that we are dealing with nothing but quartz. Now, the question comes, is there any advant age to photographers in using such a material as quartz. Let us first see the extent of the spectrum. By placing a card which has been washed over with quinine in the ultra-violet part of the spectrum, you are able to see these ultra-violet rays glowing with a pale blue light, and you will notice to what a great length these rays reach beyond the ordinary visible point of the spectrum. Now, by placing a piece of glass in front of the slit, you will see that the ultra-violet spectrum is very much shortened; in other words, the glass has absorbed these rays. I may repeat the experiment with a card which has been brushed over with paraffin oil, and the same result holds good. I have here a photograph of the electric arc taken in another manner, to which I shall have to direct your attention presently. The light in this case has to pass through no glass whatever. The spectrum was taken by a diffraction apparatus ; for the top part of the spectrum a glass was interposed in front of the slit, and we see the difference there is in the spectra, owing to the use of glass in one case and not in the other. The glass ap parently cuts off many useful rays ; but I will now draw your attention to the solar spectrum taken in the same way, in which there has been a glass placed in front of the slit for one spec trum, and not in the other. Both spectra, practically, reach the same limits. We now can answer as to whether it would be advisable for photographers to use quartz lenses for ordinary photographic purposes or not. Recollect that every ray of light you saw fluoresce on the screen is useful for photographers when they are using a light such as we have in the electric light. You will see, then, from that, if the electric arc light was usu ally employed, all those rays which are cut off by the glass could not be utilised by them, and, therefore, there would be so much power wasted. Now photographers, as a rule, do not work with the electric light, but with sunlight; we have seen that in the solar spectrum taken under similar conditions, the glass practically cuts off none of the ultra-violet rays: the atmosphere of the earth, or of the sun, or both, cuts off the extreme ultra-violet rays before the light reaches us. We there fore come to the conclusion that, so far as photographic work with sunlight is concerned, there would be no advantage in using a quartz lens over the ordinary photographic lens. Some years ago, . Mr. Claudet made an agate lens, which he considered would give him greater advantages over the ordinary photo graphic lens, simply because he could utilise the ultra-violet rays, but I think you will see from this there is no advantage in using such a lens. Remember, however, if you are photo graphing the spectrum of the electric light, or using it for illu minating a sitter, there is a very great advantage in using quartz. We may use another definite chemical compound in the shape of Iceland spar. I have here a very beautifully worked prism of Iceland spar, which has a definite composition of calcium and carbon, and I dare say we shall reach very nearly to the same ray limit as we did in the quartz. experiment. Iceland spar holds an intermediate position between quartz and glass. It was with such a prism as that that Dr. Huggins took his famous star spectra, and I thought it might interest you to throw one or two of these on the screen. They are very small, but the definition is very beautiful. Many of the black lines in these spectra indicate, probably, hydrogen. It remains to be seen whether Dr. Huggins has attained any advantage in using Ice land spar instead of glass, for if the ultra-violet stellar light is absorbed, as with sun-light, no advantage would be gained. I may mention that he gives the composition of the stars by reference to the spectral lines of well-known elements. One more point is this : Would it be advantageous to use a mirror instead of a lens ? There is a great deal to be said about this, particularly in spectroscopy, where we have to examine everything minutely. The material we utilise most easily in the case of a mirror is silver ; that is to say, we get a glass mirror, and silver it on the front surface. Now the question is, does the silver reflect every ray in the same way that quartz would transmit it ? Here I have a photograph which should give an answer to that question. The bottom half of the spectrum was taken as reflected from a quartz surface, the top half of the spectrum was reflected from a silver surface, and you will see that at one certain part of the latter the rays are very nearly absent, though beyond that again they are present. Where those rays are wanting is just at the end of the solar spectrum, and therefore, when using sunlight, it is no great advantage to use a quartz reflector over a silver reflector. In spectroscopy it is necessary to know exactly the qualities of all the substances with which you are dealing. One question in photography and in spectroscopy is, what width of slit you would use—what slice of light you would allow to pass through ? Here let me give you a demonstration. In the centre of this black disc there is a fine line of light, and there is a micrometer screw by which we can tell how' many thousandths of an inch wide it is. As a rule, about wlo of an inch is the dimension used for ordinary work. I have been referring to the photographs to two spectra on the same plate, and I must show you how it is managed. For this purpose, it is necessary to have an adjunct to the slit, and that is a shutter which is able to cut off half the slit at one time, and afterwards leave that part open, and close the other half already used. By this means we can get one spectrum adjacent to another. In comparing spectra of different metals with each other, we are able to tell whether we have any two lines co incident one with the other. (To be continued.) THE CRUISE OF THE CliYLGN, 1885. A VOYAGE to the Spanish Main with a Camera. by NORMAN may.* The Azores, or Western Isles, a group of nine islands in the Atlantic Ocean, about equidistant from Europe, Africa, and America, were discovered in 1431 by Gongalo Velho Cabral, and are a possession of Portugal. Viewed from our anchorage, just inside the breakwater, the chief town of St. Michael’s, Ponta Delgada, wore a very picturesque aspect. While lacking the masses of bright coloured flowers and bold peaks of Funchal and neighbour hood, the variety of colours with which the red-roofed houses were decorated, the blue-tiled fronts, and a curious municipal gate on the quay, gave an air of brightness and originality to the town. Rising behind were the abrupt but cultivated hills, their patches of green mapped out by tall hedges and walls. At seven o’clock I went on shore to make arrangements * Continued from page 526