The Dictionary Of Photography

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

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From this it is obvious that the greater the thickness of atmo-sphere through which the sun's rays have to pass, the greater the absorption of the chemical or more refrangible rays. The chemical power of the sun's light is, moreover, dependent on the absence or presence of dust, the height of the barometer, etc. Again, the wind has some influence, because when a dry north-east or east wind prevails, and prevents the condensation, or rather the increase, of the aqueous vapour, the chemical intensity of the light will increase; on the other hand, when a wet south-west or west wind prevails, the increase of the aqueous vapour will lower the intensity. Clouds, we all know, play an important part in determining the duration of exposures, but there are clouds and clouds. Thin haze or mist, such known as u heat mist," lowers the chemical power enormously; yet optically there is little difference. The fine white masses of clouds floating in a blue sky rather increase than decrease the power of the light, in that they act as enormous reflectors of the sun's light. Generally* however, when the sky is cloudy and the sun not actually shining, the exposure is about three times that with the sun out; with a dull leaden pall of cloud the chemical intensity of the light may be reduced to one-fourth or one-eighth, or even more. The fol-lowing table by Dr. J. A. Scott may be useful: -