The Dictionary Of Photography

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

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Contrast
Copal
Contrast. The adjustment of the light and shade and objects in a photograph so that they may give due emphasis the one to the other.
Convex. The opposite to concave - /.<?., shaped on the exterior into a spherical or round form.
Cooling and Freezing. In warm weather it is often desirable to cool liquids, such as developers and washing waters, and to the carbon printer some means of cooling is occasionally almost essential. Bottles containing the liquids to be cooled - the half gallon bottles known as Winchester quarts are convenient - are wrapped round with two thicknesses of wet calico, this being tied on, and placed where there is a strong draught of air. The dampness of the covering must be maintained, which can con-veniently be done by standing the bottles in a shallow tray containing a little water. When ice can be obtained, it is generally more convenient to cool water by leaving a few lumps of ice in it, and a similar course may be followed with other liquids, when the addition of a little water is no disadvantage; otherwise the plan of standing the bottle in a mixture of ice and water may be adopted. In special cases one of the following freezing mixtures may be used: -
No. i.
Crushed ice or snow ... ...        ...      2 parts.
Common salt ... ... ...        ...      1 part.
Will reduce the temperature from about +  io* to  - 17 or 180 C.
No. 2.
Crystallised sulphate of sodium...... 8 parts.
Commercial hydrochloric acid ... ... 5 ..
Will give about the same reduction as the above.
No. 3.
Crystallised phosphate of soda ...... 9 parts.
Dilute (1 to 3) nitric acid...... ... "4 . ..
Will reduce the temperature from about oo to - 28 or 290 C.
Copal. A resinous substance obtained from Madagascar, China, Africa, and America, and the product of several widely differing trees. It is also found as a fossil. It occurs in pale
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