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222 THE PHOTOGRAPHIC NEWS. [March 20, 1891. and the star images come up together, and if there be any local shift in the film a method of correction is supplied. The development of a plate, also, is not without its diffi culties. If there be no bright stars on the plate, and there are many spots of two degrees square on the heavens in which the eye cannot detect a single star, the growth of the picture is a matter of guesswork, and generally decided by the discolouration of the plate. It has been promised that the whole question of the preparation of the most suitable and most sensitive emulsion, as well as develop ment and subsequent preservation of the negative, should be threshed out under the auspices of the well-known photographer, Dr. Eder. The results of the experiments, which, it is understood, have been carried on in his laboratory, have not yet been published, but are looked forward to with interest. Among other difficulties in development there is the old question of the adequate treatment of violent contrasts in light. If you have a star of the first magnitude, and one of the sixth, you have in the former one hundred times the light in the latter, while, if the range extends through twelve magnitudes, the light of the brightest star is to that of the faintest as unity to sixty thousand. Therefore, there must be stars both violently over-exposed and under-exposed, all to be developed together, and the images often in contact. The effect of this violent contrast will be appreciated when we say that the images of the brighter stars are usually seen reversed, that is to say, the centre is white instead of black. The manipulation and the subsequent discussion of a single plate does not seem to offer insuperable difficulties, but it must be remembered what a very small portion of the entire sky is a district of two degrees square—the portion which is represented on a single plate. On a globe there are approximately 40,000 square degrees, so that ten thousand plates will be necessary, and since—to secure accuracy, and to be certain of the elimination of untoward specks and spots on the plates which have much the appearance of stars, but are not—it is proposed to cover the whole area of the heavens twice, twenty thousand plates will be required. To some large photo graphers the manipulation of this number of plates may not seem a very extraordinary undertaking, but then it must be remembered that the exposure required for the production of each plate is much longer than anything to which they are accustomed. In order to secure impres sions of the stars of the faintness thought necessary by those whoareresponsibleforthescheme, anexposure of pro bably fifteen minutes for each plate -will be required, while, to carry out the grand scheme in its integrity, exposures of probably not less than ninety minutes will be necessary. When it is farther remembered that these exposures can only be made at night, and when the sky is transparent, it will be admitted that astronomers are courting a suffi ciently onerous task, but one which, it is hoped, they will be able to carry to a successful issue. It is altogether premature to guess at even the number of stars which will record themselves on the sensitised films, but one thing is certain, all stars of a definite degree of brilliancy will be secured. There is no possi bility of a photographic plate overlooking or neglecting to record the position of a star, though, of course, in the discussion of the plates afterwards, human infirmity may and will fail to describe all the facts recorded on the film ; but the negative remains for re-examination and re-dis cussion, and much negative evidence is effectually ex cluded. The places of all the stars depicted on the plates, obtained with the shorter exposure mentioned above, it is hoped to form into a catalogue, which may possibly grow to the dimensions of a small library. But the results from the longer exposure must be left in a chart-form, from which astronomers of the future must pick out the precise facts for which they are seeking. Oxford University Observatory. A RAPID SENSITIVE SALT FREE FROM SILVER. Little in accordance with scientific custom is it to publish much about new results when the means of arriving thereat are kept secret for commercial or other purposes, but having the trustworthy assurance of Mr. Frederick Varley that, in the experiment which he performed at the close of Professor Meldola’s lecture at the Society of Arts last Monday night, there was no silver salt in the sensitised paper or in the developer, or in any of the solutions he used, and that the materials employed were all excessively cheap, we append a few particulars. Mr. Varley said that the process was the result of researches made by Mr. Friese Greene. Mr. Varley exposed some paper sensitised with the salt under a negative to the diffused gas-light of one of the reading lamps, in which a great amount of the light, to the extent of at least forty per cent., was cut off by having to pass through an opalescent shade and dish ; under these con ditions an exposure of about half a second was amply sufficient to secure a good picture full of detail in the shadows. We are informed by Mr. Varley that in some recent experiments he has made in conjunction with Mr. Greene, he has found means to render this salt four times as rapid. He says that had the exposure last Monday been made to a naked gas flame, one-fifth of a second would have been sufficient. By Mr. Varley’s additional dis covery, he claims that under like circumstances he could have given one-twentieth of a second. Some months back we made some mention of the discovery of this new sensitive salt, and have in our possession a print of a doll taken by its use at that time. The Photographic Club. —‘March 25th, lantern night; April 1st, adjourned discussion on “ Shutters.” Bank Holiday outing at Leigh, near Southend. Aluminium.—Among the uses of aluminium suggested by Mr. Eugene H. Cowles, according to Modern Light and Heat, are the following:—At 50 cents per pound, the new metal will compete with copper at 17 cents, the latter being 3’56 times as heavy as an equal bulk of aluminium ; but the electrical conductivity of aluminium, that is, 98 per cent, pure, is only 75 per cent, that of copper, so that one-third more area would be required to do the same work. A reduction of 45 per cent, in weight of motors for electric cars can be secured by using the new metal, which in itself is no small advantage, seeing that the latter promise to come into extensive use in the near future. The coating and lasting qualities of aluminium far surpass those of tin, and it will cover three times as much surface for equal weights, making it necessary to sell tin at 16 cents per pound in competition with the other at 50. Nickel at 70 cents would no longer be used for plated ware or coinage, the new metal being much cheaper and cleaner. He expects to see it sell at 200 to 300 dollars per ton, and at these figures it will be the cheapest metal next to iron and steel. The price must fall lower and lower as the facilities increase for making the material, and the market adapts itself to the absorption of larger quanti ties of the new metal.