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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. 1298, July 20, 1883
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- Titelblatt Titelblatt I
- Register Index III
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454 THE PHOTOGRAPHIC NEWS. [JVLY 20, 1883. lamp, which latter equals 9-5 standard candles ; consequently M. Giroud’s equals 0’95. These results appeared so very satis factory that it induced me to experiment with his standard with a view of adopting it for my purpose. The next thing was to determine its suitability; for this purpose I selected two batches of gelatine plates that I knew to vary considerably in sensitive ness, and, having a rough screen giving different intensities, I exposed them so as to obtain an equal minimum tint upon each; the ratio of exposure I found to be as 1 : 48. I have here one from each exposed in the camera for one and forty-eight seconds respectively. A collodion plate treated at the same time did not agree in the same manner ; this, I think, can be accounted for when we consider that the collodion is sensitive to rays of different refrangibility, and as the two sources varied in compo sition, the percentage of actinic rays being less for collodion in the case of gas, and greater for the gelatine, which naturally disturbed the ratio when compared with solar light; and this must be also taken into account when great accuracy is required —when comparing films of different composition, such as a bro mide with a bromo-iodide or bromo-iodide chloride; also the condition of the atmosphere when employing solar light—as, for instance, one would find considerable variation in the absorption of the solar rays, when comparing an Italian sky with a genuine London fog, with its full charge of butterine, or Blackpool, with its soot-charged atmosphere; and further, these remarks apply equally well to the phosphorescent unit, or any other source of light differing in composition from that you ultimately intend to use with the films. May I venture to suggest that the diffi culty can be overcome by the use of a constant for each film of that particular composition, in reference to the light employed when comparing, to that you intend to use with the films. On the other hand, with films of uniform composition my experience is that their ratio is not disturbed, however much the lights may differ in composition among themselves. The last experi ment having looked somewhat hopeful, I proceeded to find the best method of using my temporary standard. My first idea was to try the effect of various exposures, each upon a different portion of the film ; otherwise, intensity, constant, time varied. I tried the disc method of exposure : an improvement upon this I found was to have a rectangular plate, about ten times the length of the film to be experimented with, and a quarter wider, arranged in guides so as to have a free fall in the direction of its length, similar to a drop-shutter, at a distance of } from the furthest end when the plate is adjusted to fall past the film, “ the latter being held behind; ” an aperture was cut, the verti cal length of which varied from unity to anything, so as to correspond to any desired ratio. Many modifications of the aper ture were tried, all of which were very disappointing. My next idea was to try the effect of varying intensity, time remaining constant, as this is much more in accordance with the conditions of the tints upon the screen of the camera which we desired to register. Whilst engaged with my microscope, one evening, which is fitted with rotary diaphragms attached to the stage by means of a short piece of tube, it occurred to me that if a piece of opal glass were placed upon the diaphragm so as to cover the hole, the light would be distributed evenly over the base of the tube. This happily proved to be correct; and to save trouble in calcu lating the areas, I mounted upon a piece of opal glass a series of pieces of perforated cardboard, each containing a different number of holes in arithmetical progression, their centres coin ciding with a series of chambers upon which the opal plate was secured. The spaces between the cardboard were covered with black varnish, so as to exclude all light except that passing through each group of holes corresponding to its little room. I found this to answer admirably, the light being controlled in proportion to the area of its windows, although I soon found the instrument had very little range; and, further, the intensities would have to be in a geometrical ratio to one another. Calculation showed that it would be necessary to have the holes very small, so as to be able to get the required number upon the limited area at disposal. For instance, the instrument I am about to refer to more fully would require 39,392 holes, the smallest having 10, whilst the largest would contain 8,120, which you can easily understand would have to be very small to get them within the limited area of 200 square mm., this being the area of the thirtieth part of a -plate ; consequently, I had to abandon the holes, and turn my attention to the construction of single openings for each chamber. It was suggested to me to cut the openings in tin-foil, which I find very convenient, 1 have here a series of tints produced by apertures in arithmetical and geometrical progression respectively, also the instruments, and a modification in which you observe two wedge-shaped openings are employed, producing two series of tints, both of which are in geometrical progression to one another. Comparing the tints of one series with any two belonging to the other, the variation would be }, as the one having the greatest range is as 6 to 9, the other 7 to 9. The next difficulty that beset me was to obtain opal glass of uniform quality, which I ultimately found to be extremely difficult ; consequently I dis pensed with it, preferring a screen formed of a sheet of white paper, such as is used for photographic prints, as you will re member formed the subject of a paper by Mr. Mucklow and myself at one of your December meetings in 1881. Here is arranged the standard in its latest form, which, as you perceive, consists of four of M. Giroud’s standards arranged in a line parallel to the plane of the screen at a distance of 25 cm., the length of the flames being regulated by means of the cross wires. Between the opening formed by the two jets near the centre the the sensitometer is placed, the plane of the instrument coincid ing with a line drawn through the flames. My thanks are due to my esteemed friend, Mr. Cromwell Varley, F.R.S., &c., who very kindly suggested to me the principles of constructing the ratio, &c.—viz, making the ratio a root of 2, the largest aperture 1 cm. in area; the depth and size of the chambers also to have a simple relationship. The dimensions adopted are the 2//2, or as 1 : 1-2599, the depth of the chambers 25 mm.,the distance from the screen, and their bases 200 square mm. Thirty of these have a range sufficient for all present requirements. Here are some of the results obtained. First, a simple exposure; second, ditto, but with a mat interposed, formed by stencilling figures and lines upon a piece of glass ; and, thirdly, with a stencil mat, which, I think, for various reasons, will be found the best adapted for general working. Series A shows the variation when using 16 and 25-candle gas respectively, being a variation in the quality of 55 per cent, above the minimum value. Mark. Instrument. r i ... s ... W .. s .. W .. s .. w .. s .. o ... W .. 9 ... S .. 10 ... W .. 11 ... S .. 12 ... W .. Power of Gas in Candles. 16 16 25 25 25 25 16 16 25 25 16 16 Reading of Instrument. Time of Exposure in Seconds. 60 60 60 60 60 30 60 30 60 60 60 50 ... 2 • ... 22 . ... 23 .. — 23 .. - 23 .. - 21 . ... 22 .. ... 20 ., - 23 .. - 24 ., ... 22 .. ... 24 ., . 23 ... 23 .. 22 ... 23 . 23 ... 28 . 23 ... 23 . 23 ... 23 . 20 ... 21 . 23 ... 23 . 20 ... 21 . 23 ... 23 . 23 ... 23 . 23 ... 23 . 23 ... 24 Series B, C, D, and D 1 show the effects of different exposures upon different films :— Reading of Instrument. Exnosura. Mark. 1. 2. Practical. Theoretical. ( ... 14 14 4 ... 4 B 2 ... 24 25 64 ... 68'8 (3 ... 28 28 180 ... 176 [1 ... 14 14 5 ... 5 C 2 23 25 ... 120 ... 127'5 3 28 27 160 ... 167'5 "1 9 9 2 ... 2 2 12 12 ... 4 ... 4'5 3 14 14 8 ... 7’7 D 4 ... 17 18 16 ... 20 5 20 20 32 ... 31 6 22 23 61 ... 77 7 25 25 128 ... 150 8 27 27 265 ... 258 ‘1 15 15 10 ... 10 D 2 19 19 30 ... 24'4 [3 ... 23 23 90 ... 87 These, you observe, do not agree with the ratio of the holes, which is owing to the variation in the area of the screen capable of reflecting light directly upon the film; this goes on in a diminishing ratio towards the smallest hole. By dividing the effective area corresponding to the largest by smaller, and extracting the 29th root, which multiplied by 1'2599 gives 1'31, the basis for calculating the theoretical exposure .which, I think, may be taken as within the experimental error. In the case of the phosphorescent unit, or when a piece of opal glass is used, the ratio given to the apertures remains good for the exposures)
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