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The photographic news
- Bandzählung
- 35.1891
- Erscheinungsdatum
- 1891
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- Englisch
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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. 1721, August 28, 1891
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The photographic news
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Band 35.1891
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200 miles or more from the sheets of Miocene trap-rock with which it is probably connected. The south-western half of Scotland, and the northern parts of England, are ribbed across with thousands of dykes, which seem to be connected with the volcanic chain of the inner Hebrides (of Tertiary age). The fissures through which such dykes forced their way were not made by the molten matter itself, but more probably were the result of violent explosions and earthquakes proceeding from a region of volcanic action. I must now conclude this paper, leaving the reader to judge if I am warranted in applying the trap-dyke theory to the lunar streaks. It certainly harmonises lunar and terrestrial phenomena, and suggests a close connection between radiating streaks, chains of lunar volcanoes, mountain ranges, and ridges or lines of hills near the volcanoes. Remarks by A. C. RANYARD. If the reader will turn to the photographs of the moon, published in Knoivledge for May, 1890, and October, 1889, he will see that the rays or streaks have not sharply defined edges, as they presumably would have if, they were trap-dykes. The rays vary in breadth, many being from twenty to thirty miles broad, with very soft, nebulous edges. The whiteness of the rays in some cases may be seen to degrade gradually from a narrow, sinuous, bright band which runs along their centres—■ see, for example, the two rays from Tycho that run across the Mare Nubium, shown in the plate published in the May number for 1890. The rays seem in no way to interfere with the forms of the craters and irregularities of the lunar surface, as we should expect to find them interfering if they corresponded to a wall of injected rock either harder or softer than the surround ing material. A good instance of a broad ray passing over craters and rough ground without affecting the forms of the craters and mountains is shown in photograph No. 1, plate No. 1, in the October number for 1889, where a strikingly bright ray radiating from Tycho passes across the rough ground to the south of the Mare Nectaris, and then across the plain and onward. Lava streams and volcanic regions on the earth are generally dark as compared with the surrounding rocks, but the light- reflecting character of these rays cannot be accounted for unless they are capable of reflecting more light than light sand stone, or even than chalk ; for the light-reflecting power of the moon, taken as a whole, about corresponds to that of light sandstone. See the often-quoted observation of Sir John Herschel, who compared the light of the nearly full moon with that reflected from Table Mountain at the Cape. Everyone is familiar with the whitish appearance of the moon as seen in the daytime. It appears like a small, whitish cloud. There are many large, dark areas upon the moon ; conse quently, the brighter parts must be relatively white as compared with light sandstone. It is true that the summits of lunar mountains and craters differ greatly as to their whiteness, but few terrestrial mountains are wholly covered with snow, and, as seen from a distance, their whiteness would depend upon the amount of rock surface and shadow intermixed with the snow. The moon, as a whole, reflects a little less than a quarter of the light reflected by fresh fallen snow. My argu ment is that the brighter patches and rays are so bright as compared with the rest of the moon’s surface that their white ness cannot differ greatly from the whiteness of snow’. Mb. E. J. Browne, of Liverpool, has hit upon the happy idea of making rubber stamps from half-tone photographs, and has sent us a specimen of his work in the shape of a profile portrait impressed at the head of his note-paper. This stamp appears to owe its origin to a coarse-screen process block. Such a stamp may be useful in lieu of signature for ephemeral documents—love letters for instance—and a still more practical use for it is indicated as a means of identifying a payee in a note payable to an individual. PHOTOGRAPHY AND THE SPECTRUM. The following remarks were included in Dr. Huggins’ address before the British Association at Cardiff last week:— A very great advance has been made in our knowledge of the constitution of the sun by the recent work at the Johns Hopkins University by means of photography and concave grat ings, in comparing the solar spectrum, under great resolving power, directly with the spectra of the terrestrial elements. Prof. Rowland has shown that the lines of 36 terrestrial elements at least are certainly present in the solar spectrum, while 8 others are doubtful. Fifteen elements, including nitrogen as it shows itself under an electric discharge in a vacuum tube, have not been found in the solar spectrum. Some 10 other elements, inclusive of oxygen, have not yet been compared with the sun’s spectrum. Rowland remarks that of the 15 elements named as not found in the sun, many are so classed because they have few strong lines, or none at all, in the limit of the solar spectrum as compared by him with the arc. Boron has only two strong lines ; the lines of bismuth are compound and too diffuse. Therefore, even in the case of these 15 elements, there is little evidence that they are really absent from the sun. It follows that if the whole earth were heated to the temperature of the sun, its spectrum would resemble very closely the solar spectrum. Rowland has not found any lines common to several elements, and, in the case of some accidental coincidences, more accurate investigation reveals some slight difference of wave-length or a common impurity. Further, the relative strength of the lines in the solar spectrum is generally, with a few exceptions, the same as that in the electric arc, so that Rowland considers that his experiments show “ very little evidence ” of the breaking-up of the terrestrial elements in the sun. Stas, in a recent paper, gives the final results of eleven years of research on the chemical elemen ts in a state of purity, and on the possibility of decomposing them by the physical and chemical forces at our disposal. His experiments on calcium, strontium, lithium, magnesium, silver, sodium, and thallium, show that these substances retain their individuality under all conditions, and are unalterable by any forces that we can bring to bear upon them. Prof. Rowland looks to the solar lines which are unaccounted for as a means of enabling him to discover such new terrestrial elements as still lurk in rare minerals and earths, by confronting their spectra directly with that of the sun. He has already resolved yttrium spectroscopically into three components, and actually into two. The comparison of the results of this independent analytical method with the remarkable but different conclusions to which M. Lecoq de Boisbaudran and Mr. Crookes have been led respectively, from spectroscopic observation of these bodies when glowing under molecular bombardment in a vacuum tube, will be awaited with much interest. It is worthy of remark that, as our knowledge of the spectrum of hydrogen in its com plete form came to us from the stars, it is now from the sun that chemistry is probably about to be enriched by the discovery of new elements. Of the physical and the chemical nature of the coronal matter we know very little. Schuster concludes, from an examination of the eclipses of 1882, 1883, and 1886, that the continuous spectrum of the corona has the maximum of actinic intensity displaced considerably towards the red when compared with the spectrum of the sun, which shows that it can only be due in small part to solar light scattered by small particles. The lines of calcium and of hydrogen do not appear to form part of the normal spectrum of the corona. The green coronal line has no known representative in terrestrial sub stances, nor has Schuster been able to recognise any of our elements in the other lines of the corona. The spectra of the stars are almost infinitely diversified, yet they can be arranged, with some exceptions, in a series in which the adjacent spectra, especially in the photographic region, are scarcely distinguish able, passing from the bluish-white stars, like Sirius, through stare more or less solar in character, to stars with banded spectra, which divide themselves into two apparently inde pendent groups, according as the stronger edge of the bands is.
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