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The photographic news
- Bandzählung
- 35.1891
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- 1891
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1706, May 15, 1891
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364 sandarac. To all who have difficulty in producing benzene matt varnish, toluol matt varnish is to be warmly recom mended for a trial. The addition of more than 40 cm. 3 of toluol would cause the separation of flaky resin. A correction might be made by the addition of an ether solution of sandarac. If one adds less than 35 cm? of toluol, the matt films become too transparent and unequal. Finally, I would mention that toluol can be obtained of Herr Adlen, 10, Bezirk, Vienna. It has a specific gravity of 0-865 at 19? C. Styx. COLOUR* BY W. E. DEBENHAM. There are various other methods, besides that of using several lanterns, of combining the effect of various colours. A very usual lecture experiment is to revolve a disc at such a rate that its various parts appear homogeneous. The disc is divided into sectors, variable in size, and each of some distinctive colour. Thus, if blue, red, and green occupy the proper proportionate spaces, and the disc is revolved whilst a bright light is turned on in front of a dark background, the effect is to produce white. Helm holtz also gives the following method. Lay two discs of the colours to be combined upon a horizontal surface. Then hold a glass plate vertically, so that the reflected image of one disc appears to cover the other disc looked at directly through the glass. This method gives us the opportunity of increasing the effective power of one or other colour according to the greater or less angle at which the plate of glass is observed. The discs should be laid on a dark or black surface ; if surrounded by white, they look dark and dingy. Another method which I pro pose to show to-night, and which has the convenience of demonstrating composition of colour by a single lantern, is to have the colours in alternate stripes, and show them in the lantern first as the two colours distinct, and then, by throwing the image very much out of focus, allow them to blend. If the lens cannot be sufficiently thrown out of focus for complete blending, a piece of white card or paper can be held so near the lantern as to intercept the rays, and be so completely out of focus as to ensure blending. The grating of red and green now shown in the lantern gives a sharp image on the screen, but by moving the lens, or holding a piece of card on the screen and advancing it to the lens, the lines of the grating disappear, and are sub stituted by a general yellow. These methods depend upon the colours to be combined falling upon the same part of the retina. I find, however, that it suffices to allow them to fall upon corresponding portions of the retina of the two eyes. The frames now handed round contain a film of green in the opening for one eye, and of red in the opening for the other eye. If a light be looked at through these spectacles, it will not be seen of the colour of either film, but as a yellow—a greenish yellow. If there were as convenient a way of modifying the illumination to either eye as there is of lowering the light in one of a pair of lanterns, it might be so arranged that the yellow should not be found to incline either to the green or to the red. In another set of these spectacles, I have made the film for one eye red, and for the other eye it is divided. The lower half is of the same green as before, and the upper half of a bluish green. On " Concluded from page 339. | May 15, 1891. looking at a light through the red and blue-green it appears white ; but a slight shifting, so as to bring the other green into use, gives yellow. Blue and yellow have been called primary colours, and by mixing these in transparent pigments a good green may be compounded. They do not, however, give green, but white, when their effect is produced simultaneously on the eye. This will be seen by allowing the blue and yellow discs now in the lanterns to overlap on the screen ; also by looking at a light through the blue and yellow spectacles shown, and in other ways. White, which, generally speaking, is a compound of all the colours, may be produced to the eye by compounding two colours, and any two colours which compounded pro duce the effect of white, are said to be complementary to each other. Starting from the red end of the spectrum, we may take any colour from that to a yellowish green, and we shall find another colour in the spectrum towards the violet which, compounded with the first, will give the effect of white. These whites are to the eye indistinguish able from one another and from ordinary white light, but may be recognised by analysis with the prism, or by hold ing coloured screens instead of white paper in the path of light. Thus, in the white compounded of greenish blue and red, violet loses its brightness, whilst red comes off badly in the white composed of violet and yellowish green. Green, not inclined to yellow or blue, requires a colour not to be found in the spectrum for its complementary, viz., purple, a compound of violet and red. If we consider what colours appear to be related and what opposed, we find that a circle may be arranged, commencing with the spectrum violet and going through the series to red, which may be made not quite to close the circle, but may be blended into the violet by purple. In fact, the red and violet, although at the opposite ends of the spectrum, do not strike the eye as being the most opposed in character, nor do they act as complementaries to each other. In the colour circle now shown, I don’t think anyone unacquainted with the spectrum would select the place between the violet and red as the starting point in particular any more than that, say, between the yellow and red or the yellow and green. This approximation in visual character of the two ends of the spectrum, and most contrasted in wave-length and in physical characteristics, may be due to the fact that the violet is approximating in the wave-length to half that of the red, and, in rapidity, double. A little beyond the violet of the visible spectrum this simple proportion holds good. Now, as each colour sensation acts not only at a definite point, but over a considerable portion of the spectrum—recognising wave-lengths, that is, that are any where near its maximum of sensitiveness—it seems possible that, as we approach the relation of double rapidity such as exists in the musical octave, that sensation nearest to that of double rapidity may be excited to some extent, and thus the relationship of violet and red as approximating to one another, instead of striking us as the most opposed, may be accounted for. On the other hand, the fact must be remembered that the vibration of actual half wave length of red is invisible. The slide next thrown on the screen illustrates the essential difference between combinations of colour sensa tions, such as have been shown, and combinations of coloured films or pigments. In the first case we have proceeded by additions of sensation ; in the present case we proceed by subtraction. In this slide three discs of THE PHOTOGRAPHIC NEWS.
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