The Dictionary Of Photography

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

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Calorific Rays of the Spectrum
factors, and the sum will be the logarithm of the number required.
Division by Logarithms. From logarithm of dividend subtract logarithm of divisor. The remainder will be the logarithm of the quotient.
Extraction of Roots by Logarithms. Divide the logarithms by the exponent of the root to be extracted. The quotient will be the logarithm of the required root.
The last column of the table, headed D, gives differences between successive logarithms, and may be used to further extend the table by interpolation. Let, for example, the logarithm of 2795 be required, half the difference is added to the logarithm previously obtained, and we get '4464. Obviously the last figure thus obtained cannot always be relied upon as being accurate to a unit.
Calomel. An old name for the insoluble mercurous chloride. When digested with dilute hydrochloric acid in a water bath - say, a jam jar set in a saucepan - it is rapidly converted into mercuric chloride, but the free access of air is essential to this reaction. Some persons who require mercuric chloride solution (corrosive sublimate) for the purpose of intensification, prefer to prepare the solution as above, as calomel can be bought without trouble, while mercuric chloride comes within the Poisons Act. An ounce of calomel requires abouta fluid ounce of the strong commercial hydrochloric acid and 12 ozs. of water, Any excess can be neutralised with ammonia.
Calorific Rays of the Spectrum. Those rays of the spectrum which produce or emit heat. They are found at the red end of the spectrum, the most powerful being situated beyond the visual rays. This can be proved in the following manner: - The rays of the spectrum are conducted through a solution of iodine in carbon bisulphide, which absorbs visual rays, but allows heat rays to pass ; these can be brought to a focus by certain means, and in this focus magnesium ribbon will catch fire, and platinum be brought to a white heat. Taking 100 as the maximum intensity of the heat rays, the following table will give the values of the colours of the spectrum as heat-producers : -