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The photographic news
- Bandzählung
- 27.1883
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- 1883
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1301, August 10, 1883
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The photographic news
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- Titelblatt Titelblatt I
- Register Index III
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August 10, 1883. | THE PHOTOGRAPHIC NEWS. 509 wood was burned or charred indicated approximately the intensity of the solar radiations. Professor Stokes has recently pointed out that it is not by any means necessary to sacrifice a whole wooden bowl for each experiment, and that a highly satisfactory result can be obtained by using a strip of card mounted on a kind of half-round gallery placed as shown by the subjoined wood-cut. vation. When this (the latitude) is known, the adjustment of the instrument may be performed at once by means of a graduated circle (D) provided for the purpose. “ Another defect of the Campbell instrument is the necessity of having the diagrams;or cards cut to different curvations to fit round the sphere at different seasons of the year, as also the necessity of shifting them from day to day. The Whipple- Casella supporter is provided with several movable card-holders, K, all adapted to receive strips of card cut straight, and of uniform width, so that they are perfectly interchangeable one with another, and the same kind may be used all the year round.. “ The card-holders are held in their place by a clip, E, so that Another improvement is the accurate graduation of the cards into hours and half hours ; and in order to render the same cards available at all seasons, the frame is so marked off that the posi tion of the printed card can be varied according to the time of the year In this form the apparatus is a sun-dial of no mean value, it being often easy to tell the time by it within ten minutes, and all calculations are rendered unnecessary, owing to the facilities provided for the accurate adjustment of the cards. “Not only does the‘sunshine recorder’in its present con venient form register sunshine, but the extent to which the card is charred .indicates in a very striking manner the intensity of this sunshine ; it being most curious to note how the course of the sun is sometimes marked by a fine line only just visible when the card is carefully placed in relation to the light, and at others by a thorough charring of the card throughout its entire thickness. The photographer knows very well that the thermic energy or the sun’s rays docs not correspond exactly with their actinic activity; but, notwithstanding this, valuable information may be obtained by an inspection of the card when an exposure is about to be made.” The following description of the somewhat more elaborate Whipple-Casella instrument, is also to be found in a subsequent number of the PHOTOGRAPHIC News -No one is more interested than the photographer in the vigour and duration of the sun’s rays that fall upon the earth, and hence we make no apology for bringing before our readers another and improved apparatus con structed upon a design of Mr. Whipple, the director of the Kew Observatory, and Mr. Casella, the well-known optician of Holborn Bars. The instrument, as our readers will see from the drawing, is an adaptation of the glass sphere which Mr. J. F. Campbell was the first to suggest as a ready means of recording the hours and minutes of sunshine. The sphere acts as a convex lens or burning glass, and is thus made to produce a charred line along a printed sheet or diagram that is placed suitably in the focus of the rays. The diagram is divided into spaces representing the hours of the day, and if at sunset it is found that a charred line exists through out all these spaces, there is proof at hand that the sun has been shining all day. In the same way, if the charred line only appears at certain intervals on the printed sheet, then we know that during certain times of the day only has the sun shone, the actual periods of dull and bright weather being at once shown by the diagram. “About ninety per cent, of sunshine may be in this way re corded, for very feeble sunlight fails to exert any action ; but we suggested in our former article that if a more sensitive recording material were necessary, this might probably be produced by treatment of the paper to be charred, with slightly sensitive chloride of silver or other similar medium. “ There are, however, some slight defects in the Campbell recorder, which have led to the design of the present improved instrument, one of the shortcomings being the fact that it is necessary to construct the instrument especially for the latitude of the spot where it is to be used. It cannot be transferred from one place to another. The Whipple-Casella instrument, on the other hand, is universal, that is to say, it may be set to work in any part of the world on adjusting the polar axis (B C) of the glass sphere (A) to the height of the pole at the place of obser- when it is necessary to change the paper, one holder is slipped out and another in, an operation occupying but a second sr two, leaving the observer at liberty to remove the cards from the holder at any time during the day when convenient, and allowing of his drying them before the fire, if wet, before withdrawing them from the grooves, thereby avoiding the risk of tearing or destroying them in the operation. “ In order to assist the observer in placing his card in the proper position for receiving the sun’s image, a graduated scale, F, is provided, engraved with divisions indicating the sun’s place any day of the year. If, then, the pointer be set to the correct date, and the clamp, G, bo screwed fast, the card will be in its correct place. “ Owing to the smaller diameter of the circle described by the sun’s image round the glass sphere at the solstices than at the equinoxes, it is found necessary to have card-holders to two differ ent diameters, the larger ones to be used during the months of May, June, July, and August, whilst the smaller ones are em ployed the remainder of the year.” These instruments rather record the heat of the sun’s radiations than the actinic energy, and are not actinometers in the strict sense of the word ; but, by reducing the light, and using chloride of silver paper, they are converted into extremely useful actino meters. FRENCH CORRESPONDENCE. Captain Bint’s Photo-engraving Process for HALF- tints. Captain Bint has finished his experiments on a photo engraving process on zinc; it may be used either for copper plate or lithographic printing. The following des cription shows this ingenious method.
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