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The photographic news
- Bandzählung
- 29.1885
- Erscheinungsdatum
- 1885
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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. 1406, August 14, 1885
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Zeitschrift
The photographic news
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Band
Band 29.1885
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- Register Index III
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Band 29.1885
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August 14, 1885 J THE PHOTOGRAPHIC NEWS. 519 an easy problem to determine exactly how much of the barometric pressure is due to the air alone, if we know how much vapour is present in it at any given time. This is easily determined by finding the dew-point, as will be subsequently explained. Now let us suppose that the barometer on any given occasion stands at 29’8 inches, the temperature being 25° C., and the dew-point being 20® C. On referring to a table of vapour tensions, we find that the pressure of aqueous vapour at 20° C-, in a space saturated by it, is -7 inch. Therefore, in the present case, we have only to find what the pressure of this same vapour would be at25°C., and substract the amount from the baro metric reading. A pressure of •7 inch at 20° C. becomes at 25° C. a pressure of— - 273+25 273+20 which is nearly -712 inch. Therefore the pressure due to the air alone is 29’8 — -712, or 29'088 inches. Thus we see that the vapour tension is always extremely small com pared with the dry air pressure, and the greater the pro portion of vapour in the air, the less dense it will be. Since the temperature and humidity of the air are con stantly changing, the barometer is in a state of continual oscillation ; and as wind is the result of differences in atmospheric pressure, these determination of these con ditions is of the utmost importance in meteorology. We shall conclude this chapter by referring to the con nection between the height of the barometer and sunshine. In a communication to the Meteorological Society in 1879, Mr. Whipple shows that the readings of the so-called weather-glasses are misleading in this respect. He found that, for Kew, the maximum sunshine appeared when the barometer had a height of 3015 inches, and that, when higher, clouds returned, so that at 30.6 the sunshine was no greater than 29'2. Thus, as Captain Abney points out, it appears that for London the most favourable weather for landscape work is when the barometer stands at about 30 inches, and then, according to Mr. Whipple’s observations, there should be, not absolutely clear sky, but just such a proportion of cloud and sunshine to give good effects of light and shade, and a rich gradation of tone. This does not necessarily apply, however, for other localities. It would be interesting, from a photographic point of view, if, by careful comparison of the sunshine records with barometric heights, it were possible to foretell the probable kind of light which might be expected, by observing the height of the barometer. (To be continued.) APERTURES FOR STOPS FOR LENSES. BY C. FEREIS. Ok page 248 of the Times, I find the scale of sizes for stops for photographic lenses which have been adopted by the Photographic Society of Great Britain, which, taken in connection with the comments, would carrv the idea that it is only necessary to know the equivalent focus of a lens to be able to make a stop that will give the same amount of illumination on the plate, whether the lens be a single, double, or triple combination. There seems to be a principle in optics which all writers on apertures for stops have overlooked ; that is, a larger bundle of parallel rays of light will pass through a given size stop after passing through a condensing lens than before, and the greater the distance of the stop from the lens, or the greater refracting power of the lens, the more light will be admitted. In the single combination, or view lenses, the stop is always placed in front of the lens, and the amount of light admitted is always the same, and with such the scale for stops referred to is all right; but when wo have the com pound lenses to deal with, with the stop placed within the combination, there can be no scale or system of stops adapted to “ equivalent focus.” A little mathematical demonstration will show the difference between the two classes of lenses. Take, for comparison, a single lens 12-iuch focus, stop, and a com pound lens 12-iuch equivalent focus, front lens 15-inch focus, stop 3 inches, back of front lens. Now it is evident that a bundle of parallel rays of light that would be con densed to a point in 15 inches would be condensed one- fifth their diameter in three inches, the place of the stop, which would be equal to a bundle of parallel rays one and one-fourth the diameter of the stop. This would give an illumination on the plate more than one and one-half (1’56) times as strong as the same stop in the single lens camera. Now move the stop back one inch farther from the front lens, or shorten the focus of the front lens to about 12} inches, and you will increase the illumination on the plate to double what you would have with the same stop in the usual place in the single view lens. The English scale is all that can be desired for single lenses ; the ratio is a good one, being such that one stop is about 1'42 times the diameter of the next size smaller, and giving just double the intensity of light on the plate. What the photographer requires is a uniformity in the numbering and size of his stops, such that the same number of stop shall give the same illumination on the plate, whatever be the combination of lens used, aud as “ equivalent focus ” will not harmonize a compound and single combination lens, the only alternative is for the optician to estimate the extent of refraction between the front lens and the stop in double combinations, and make and number the stops to correspond with the adopted scale.—Photocjraphic 1 imes. ebiew. Die PHOTOGRAPHIE mit Chloe-Silber Gelatine. Von Ludwig David und Ch. Scolik. {Vienna: A. Hartleben.) A WELL-WRITTEN and compact hand-book of the gelatine process, and consisting of a hundred and fifty large octavo pages. The book bears the impress of being written by practical workers, and as it contains much condensed information, is likely to be of service to all who are engaged in photographic work. Hotes. With what persistent ingenuity enterprising photo graphers turn to account the publication of “ births ” in the limes, we described last week. But it is not the “ hatches ” only which are made an excuse for touting for business. The “ matches ” and “ despatches ” are also watched with similar care, as everyone who perpetrates a marriage, or loses a relation, and subsequently makes the fact public in the newspapers, has only too good reason to know. The photographic copy of the list of marriages is usually mounted on an appropriately fancy card, whilst, in the case of the deaths, a mourning-bordered card, ornamented with the usual insignia of the tomb, is employed. Nor is this all; for whilst the Benedict is often inundated with offers to photograph the wedding-cake, the brides maids, and the marriage presents, the mourner has his grief aggravated by batches of sample photographs of
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