The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Decomposition of Light                                           Density
Decomposition of Light. White light on passing through a prism is decomposed or separated into its constituent rays. All lenses being formed on the principle of a prism, it is evident that light passing through a lens would be decomposed and give rise to chromatic aberration, but this is obviated by combining a lens of different shape and glass, so as to recombine the scattered rays. (See Chromatic Aberration, and Lens.)
Definition is the accurate concentration by the lens of the light from a point in an object to the corresponding point in its image without spreading to adjacent parts. Perfection of defini-tion depends chiefly on the characteristic curves of the lens, on the composition of the glass employed, the relative positions and forms of the surfaces and their proper grinding, the cen-tring of the elements of a combination, and, in a doublet, the centring and due separation of the combinations. (See Lens.)
Deliquescence. The liquefaction of a highly soluble salt by the absorption of water from the atmosphere.
Densitometer. A contrivance of Mr. Dawson, and introduced by the firm of Houghton, for estimating the printing density of any negative. A dense portion of the negative is held up to a source of light and a screen illuminated by the transmitted light is compared with a screen illuminated by light from the same source which passes through an adjustable diaphragm.
Density - literally opacity ; and in this sense correct density is an attribute of a good negative. It should be just sufficient to give due relation to the shadows, and yet allow the detail in the high-lights to print. Almost all plates differ in the value of the deposit of metallic silver, of which density or opacity is formed; and the correctness of the judgment necessary in this particular is one of the best tests of a good worker - it can only be obtained by experiment with every brand of plate used. With some the development must be pushed till the high-lights just show on the back of the film, and the whole surface of the plate is becoming blackened ; whilst with others, especially those containing iodide or those having a film rich in silver, the test of the blackening of the surface of the plate will usually be sufficient. The colour of the deposit of silver affects the result
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