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Page:Popular Science Monthly Volume 90.djvu/270

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How Are Photographs Made in Natural Colors?

Nearer and nearer is the camera coming to perfect representatiftn of the blush of the rose

By A. Press

��ALL photography depends on the fad- ing of colors when exposed to a strong light. Of course, when a colored body fades, it would not be reasonable to suppose that it still had the precise chemical composition or structure that it had before fading. It is possible in many cases to magnify or accelerate the fading efifects by steeping the partly faded object in a suit- able solution. In photography this process is called developing. After the develop- ment it is only necessary to dip the faded silver bromide plate or paper into a solution of "hypo" which dissolves the unfaded silver bromide and leaves the black or faded variety of silver to give the finished negative.

Such a negative, however, is not a colored photograph, because silver bromide always fades to black. If a mixture of chemicals could be found in which the components of the mixture faded individually to difTerent colors, then color photography would be a fact and not an experiment involving subterfuges.

One difhculty has been that silver bromide and its allied salts are found to fade with practically only blue and violet rays of light. Hence, an ordinary photo- graph gives an untrue represent ation of a colored scene, be- cause the photographic plate is blind to all other colors than the blue and violet. Ways have more recently been dis-







The Gurtner method of obtaining two-color effects by means of blue and yellow patches of color on the plates. It is also possible to obtain patches of white and a fairly good substitute for black by this method

��covered of making silver bromide plates sensitive to all colors, but the chief difficulty now is that the silver salts so treated always fade to the same black color. Thus the great problem in color photography to-day is how to give a different color or tone to the different sets of silver grains faded by the differently colored rays occurring in a composite colored picture.

An exceedingly simple and ingenious method of two-color photography has been devised by Gurtner. Two grades of plates are required. One is a lantern-slide plate, which must be previously stained by means of a napthol orange solution. Such a lantern-slide plate is generally very slow. This plate is then placed face to face with a panchromatic plate in an ordinary camera and exposed through the glass of the lantern-slide plate.

Turning to the illustration, it will be seen that a yellow patch in the chart photographed will send out yellow light rays that do not fade the silver of the lantern-slide layer because only blue or violet rays can affect it. Such yellow rays, however, in passing through into the pan- chromatic plate can affect the specially treated silver forming the sensi- tive surface of the plate. On the other hand, the blue patch in the chart affects only the lantern film. It cannot pass through to the panchromatic film, since, by staining the lantern film with an orange yellow dye, it effectively stops all blue or violet light from passing through. With the white light again, because it really has in it a mixture of yellow and blue rays, it affects both plates in the camera, whereas the black • patch in the chart affects neither of the corresponding por- tions of the plate. Thus all the blue patches of the chart are photographed on one plate and all the

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