The Dictionary Of Photography

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

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Exposure
The following table by Dr. Spitaler may be regarded as sup-plementary to the foregoing, as it will enable Dr. Holetschek's results to be applied with sufficient accuracy to other degrees of latitude: -
Spitalers Table of the Chemical Intensity of the Light for every ten degrees of latitude, for the northern hemisphere, and the middle of each month. This may be used for the southern hemisphere by altering the time of year; thus, January in the place of July, etc.
The variation of light during the day is a very important matter, and practice seems to have proved that the most intense light is on a cloudless sunny day about 11 to 12 in the morning; after noon the intensity sinks, because the heat of the sun has filled the air with aqueous vapour and the sun itself has begun to sink, and therefore its light, passing through a greater and ever-increasing thickness of atmosphere, loses in a marked manner its chemical activity, which resides, as we have already seen, in the blue and violet rays, which are more quickly, and in greater proportion, absorbed than the less refrangible rays. Professor Langley (Washington, 1884) has given the following results of the experiments carried out by him, as to the absorption of the different rays of the spectrum.