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The photographic news
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- 35.1891
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- 1891
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- Bandzählung
- No. 1721, August 28, 1891
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The photographic news
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612 THE PHOTOGRAPHIC NEWS. [August 28, 1891. towards the red or the blue. In such an arrangement the sun’s place is towards the middle of the series. At present a difference of opinion exists as to the direction in the series in which evolution is proceeding, whether by further condensa tion white stars pass into the orange and red stages, or whether these more coloured stars are younger, and will become white by increasing age. The latter view was suggested by John stone Stoney in 1867. About ten years ago Ritter, in a series of papers, discussed the behaviour of gaseous masses during condensation, and the probable resulting constitution of the heavenly bodies. According to him, a star passes through the orange and red stages twice, first during a comparatively short period of increasing temperature which culminates in the white stage, and a second time during a more prolonged stage of gradual cooling. He suggested that the two groups of banded stars may correspond to these different periods—the young stars being those in which the stronger edge of the dark band is towards the blue ; the other banded stars, which are rela tively less luminous and few in number, being those which are approaching extinction through age. In the spectra of the white stars the dark metallic lines are relatively inconspicuous, and occasionally absent, at the same time that the dark lines of the hydrogen are usually strong and more or less broad upon a continuous spectrum, which is remarkable for its bril liancy at the blue end. In some of these stars the hydrogen and some other lines are bright, and sometimes variable. As the greater or less prominence of the hydrogen lines, dark or bright, is characteristic of the white stars as a class, and diminishes gradually with the incoming and increase in strength of the other lines, we are probably justified in regarding it as due to some conditions which occur naturally during the progress of stellar life, and not to a peculiarity of original constitution. To produce a strong absorption-spectrum, a substance must be at the particular temperature at which it is notably absorp tive ; and further, this temperature must be sufficiently below that of the region behind from which the light comes for the gas to appear, so far as its special rays are concerned, as darkness upon it. Considering the high temperature to which hydrogen must be raised before it can show its characteristic emission and absorption, we shall probably be right in attri buting the relative feebleness or absence of the other lines, not to the paucity of the metallic vapours, but rather to their being so hot relatively to the substances behind them as to show feebly, if at all, by reversion. Such a state of things would more probably be found, it seems to me, in conditions ante rior to the solar stage. A considerable cooling of the sun would probably give rise to banded spectra due to compounds, or to more complex molecules which might form near the condensing points of the vapours. The sun and stars are generally regarded as consisting of glowing vapours surrounded by a photosphere where condensation is taking place, the tem perature of the photospheric layer, from which the greater part of the radiation comes, being constantly renewed from the hotter matter within. At the surface the convection currents would be strong, producing a considerable commotion, by which the different gases would be mixed and not allowed to retain the inequality of proportions at different levels due to their vapour densities. Now the conditions of the radiating photosphere and those of the gases above it, on which the character of the spectrum of a star depends, will be deter mined, not alone by temperature, but also by the force of gravity in these regions ; this force will be fixed by the star’s mass and its stage of condensation, and will become greater as the star continues to condense. In the case of the sun, the force of gravity has already become so great at the surface that the decrease of the density of the gases must be extremely rapid, passing in the space of a few miles from atmospheric pressure to a density infinitesimally small ; consequently, the temperature-gradient at the surface, if determined solely by expansion, must be expremely rapid. The gases here, however, are exposed to the fierce radiation of the sun, and unless wholly transparent would take up heat, especially if any solid or liquid particles were present from condensation or convection currents. From these causes, within a very small extent of space at the surface of the sun, all bodies with which we are acquainted should fall to a condition in which the extremely tenuous gas could no longer give a visible spec trum. The insignificance of the angle subtended by this space, as seen from the earth, should cause the boundary of the solar atmosphere to appear defined. If the boundary which we see be that of the sun proper, the matter above it will have to be regarded as in an essentially dynamical condition—an assem blage, so to speak, of gaseous projectiles for the most part falling back upon the sun, after a greater or less range of flight. But in any case, it is within a space of relatively small extent in the sun, and probably in the other solar stars, that the reversion, which is manifested by dark lines, is to be regarded as taking place. THE PHOTOGRAPHIC CONVENTION OF THE UNITED KINGDOM.* Abstract of Report of the Delegates of the Photographic Club. The sixth Photographic Convention of the United Kingdom was opened by the Mayor of Bath in the Guildhall of that city. After a speech of welcome from him, Mr. Bothamley, the retiring president, returned thanks, and alluded to the fact that that city was intimately associated with the early history of science and photography by the names of Fox Talbot and Herschel. He then vacated the chair in favour of his successor, Mr. William Bedford. The President, who was received very cordially, delivered his presidential address. He referred to Bath as of special interest to photographers, owing to its vicinity to Lacock Abbey, the first spot in the kingdom depicted by means of photography by Fox Talbot. After a remark on the increasing number of societies, the spread of literature and journalism, and the success of recent exhibitions, he alluded to the scheme for the formation of a Photographic Institute somewhat similar to those already existing at Berlin and Vienna. He also spoke of the new premises of the Camera Club, and in connection therewith of their annual conference and one-man exhibitions, and of the photographic survey now being carried out by the Birmingham societies. He called attention to the investigations by Messrs. Hurter and Driffield on the effect of development on the ratio of densities of nega tives ; to the report of M. Lippmann’s discovery of a means of reproducing and fixing the colours of the spectrum ; and to Colonel Waterhouse’s process for obtaining a reversal of the image by the addition of thiocarbamides to the eikonogen developer. In optics the latest novelty was the lenses recently introduced by Zeiss, upon which Dr. Eder had so favourably commented. The primuline process of Messrs. Green, Cross, and Bevan, and the diazotype process of Feer were then dwelt upon, as well as Mr. Carey Lea’s further experiments with allotropic forms of silver. After treating most exhaus tively of the scientific discoveries and processes connected with photography, he dealt with the advance made in the artistic direction, not forgetting to comment on the new school, which, notwithstanding the renunciation of its former master, is still living, and, what is more, reaping due recognition of its merits. Even automatic photography was included in the address of our President. On the following day, Tuesday, the excursion to Tintern and Chepstow took place. On the Wednesday, after the general meeting and the meeting of the general committee, the group of members was taken by Mr. Ashman, and, notwithstanding the rain, resulted in a most satisfactory picture. The afternoon and the evening of that day were devoted to papers, of which we will give but the briefest outline, the full text having been published in all the journals. The subjects which occupied the council and general com mittee meetings were, principally, the place of meeting for next year’s convention, and the affiliation scheme. Regarding the former, of the various invitations sent for the 1892 gathering, the one for Edinburgh was accepted. As to the affiliation scheme, it having been brought forward only then, it was thought that the members were scarcely in a position to * Bead at a meeting of the Photographic Club, August 12th, 1891.
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