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The photographic news
- Bandzählung
- 27.1883
- Erscheinungsdatum
- 1883
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- Englisch
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- F 135
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1271, January 12, 1883
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Zeitschrift
The photographic news
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Band
Band 27.1883
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- Titelblatt Titelblatt I
- Register Index III
- Ausgabe Ausgabe 1
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- Ausgabe Ausgabe 641
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- Ausgabe Ausgabe 753
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- Ausgabe Ausgabe 801
- Ausgabe Ausgabe 817
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Band
Band 27.1883
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the two first spaces, or in the three first, or four first, or all five, the result was the same, and henc. there could be no question as to the justness of the award. If only the first space is taken into consideration, we may of course sometimes encounter the result the Athenians got; but whether or not personality is traceable therein, it is very certain that the sum cf the other spaces would give a most satisfactory reading. At the same time the voting in space No. 1 must of course have a true value set upon it. This could be done in a variety of ways. We do not know how the matteri was managed in Edinburgh, but the spaces might each of them have a specific value, which gradually decreases. If ten medals were given, the first space might count ten, the second nine, the third eight, and so on, when it would be an easy matter, by adding up the different votes given to a certain picture, to estimate the value in which it is held by the members of the society. The total would express this at once ; and if necessary, there might be a provision that no picture obtaining less support than that of half the members voting should have any medal at all. The advantage of the system is that the judging of pictures becomes impersonal instead of personal ; the character of the judges could no more be assailed than the verdict of the ballot papers. Further, all members of a society—the quiet unobtrusive individuals, as well as those who are generally to the fore—would have a voice in the judging, and hence it is likely to be as far as possible free from human error. ( ( N 2» r ) 2n ) ) )1 _ sin 2(i—1) I + ! 1 _tan2i—r) ( >x; | sin ‘(+r) J ( tan 3+" ) ) where i is the angle of incidence on the prism, and r of refraction, and n the number of prisms. The third column was obtained by dividing the intensities by the relative dispersions. So you see that with 10 prisms the intensity of spectrum is very small. With compound prisms this intensity may be increased for the same dispersion ; but, in my own experience, the definition is never so good as with simple prisms. Now the intensity of the resulting spectrum is evidently proportional to the face of the prism, that is, without taking into consideration the slit. Now a prism of two inches projected face is a large prism, and thus four square inches may be taken as the section of the beam of light forming the spectrum ; and this beam of light, when arriving at the last of the ten prisms, may be measured by 4X1 =*4, calling the original beam 4. Let us take this intensity of light, and compare it with a 1} inch square face diffraction grating of 17,200 lines to the inch. A grating gives a number of spectra on each side of a central image. For practical purposes, we may take it that the central image reflects one-third the light, while the other two-thirds is distributed amongst the different spectra. The first pair of spectra on each side of the central image takes up about half of that which remains ; so that one spectrum of the first order has in it about of the original light, the next spectrum to it about , and the third about 35, the remainder being distributed amongst the spectra of higher orders (fourth, fifth, &c.), being the same intensity of light. We find, then, that the intensity of light for the third order may be represented by No. of prisms. The second column was calculated by Fresnel’s formula • Read before the Edinburgh Photographic Society. 1 2 3 4 5 10 allows, besides the visible spectrum, the latter, but not the former, to be transmitted. Can glass, rock salt, &c., be done away with ? I have shown in my Bakerian Lecture how by a system of three reflections from silvered surfaces it may be avoided, but the practical difficulties of the plan are such that a man must be trained in patience to meet with success. Three months ago I received a paper from Prefessor Rowland, de scribing a grating ruled on a concave surface, and entering into details of what such gratings would do. I must confess I was sceptical, and imagined that perhaps the sketch was too rosily coloured by the inventor of these gratings. One day in October Professor Rowland walked into my laboratory, and told me he had come from America, and had brought me a grating exhibit ing a certain peculiarity, which was that it had only one bright spectrum and all the others dull, and said that this one spectrum should be useful in my particular work. He came again, and brought other gratings, with the result that he left me three—> Intensity of spectrum at angle of minimum deviation. . 1 . -401 . *169 .. -070 . -029 ... '00025 Intensity of light passing through prisms. ••• '825 ... - '677 ... — '561 ... - '467 ... ... '391 ... ... *105 ... A 121X3= 4X4X3=286, or about "045. Now the third order corresponds as nearly as possible, for the blue, to ten prisms of 60° ; so you see that by using the grating there is a very apparent loss in light. Mr. Christie has found, however, that the loss of light in passing through prisms is more than half that penetrating ; so that, in reality, the lights are more nearly equal. I heve said that the dispersion for the blue is equal to about ten prisms, but for the red part it is equal to about forty prisms, so that here we have an enormous gain in light in using the grating. The diagram, page 22, is a wave-length map of the B line, which is about halt way down the red of the spectrum. The small figure on the bottom right-hand side shows it as obtained by prisms. The map was made from photographs taken with the Rutherfurd grating, with the second-order spectrum. The pris matic photograph was taken with three prisms. It will thus be seen what a gain in resolving power there is in using a grating for rays of low refrangibility. I would call to your recollection the plan adopted in using a spectroscope : 1st, we have a slit, and a collimating lens to give parallel rays ; 2nd, the dispersion apparatus ; and, 3rd, a camera or telescope with one lens or two respectively : in other words, glass intervenes. Now glass will cut off rays at each end of the spectrum—in the ultra-violet and infra-red ; therefore, in deli cate research in these regions the aim of physicists has been to do away with glass as much as possible, or to substitute some thing for glass which would allow all rays to pass through. Un fortunately no medium allows all rays to pass. Iceland spa and quartz, for instance, allow the visible spectrum and the extreme ultra-violet to pass, but not the infra-red ; whilst rock salt ON THE ROWLAND DIFFRACTION GRATING. BY CAPTAIN w. DE W. ABNEY, R.E., F.R.s.* 1 It may be interesting to the Photographic Society that I ‘ should show them, not a new instrument, but an instrument that has been so improved as to be increased in value at least 100 per cent. The members may be aware that, when studying the spectrum, whether for the analysis of vapours or for researches in photography, the spectrum is ordinarily produced either by passing a beam of white or other light through a slit of the width of perhaps 10th of an inch and then through one or more prisms, or else by allowing it to fall through what are called " gratings” (i. c. t flat surfaces ruled with very close lines), which gives rise to the phenomenon of diffraction. Each method has its advantages; the prism compresses the red end of the spectrum, and extends the violet; whilst, the grating widens out the red end, and compared with the prismatic spectrum, con denses the violet end. For measurement the grating is to be preferred ; but, till recently, the brilliancy of spectrum as furnished by prisms was considered so important, that prisms were generally used. Under some circumstances this is, no doubt, still the case. But I should like to point out what these circumstances are. If you make a beam of light impinge on a prism, besides the spectrum, you will find that there is a reflection of white light from one surface, and also a reflected spectrum. Roughly speaking, only 85 per cent, of the light falling on a prism of G0° at the angle of minimum deviation finds its way to the second surface, and the same percentage of that percentage only finds it way out. If you increase the number of prisms to two, this last percentage must be multiplied by itself, to find the light coming through the second prism, and so on. I give a table which shows with greater accuracy how much of the light of the refrangibility for which the prism is set, to give the minimum deviation, penetrates.
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