271 
Illumination and Vegetation. 
■of our total light go through a diurnal and annual rhythm of 
intensity, and the precise form of the curves expressing these 
variations is different for every degree of latitude from equator to 
pole. Different parts of the earth have thus a special climate as 
regards light just as in regard to temperature. 
Were clouds and mist absent, or uniform in their occurrence 
the light-climate of places could he calculated from the known 
■elevation of the sun at different hours and the absorbing and 
scattering power of the different thicknesses of atmosphere super¬ 
posed upon places at different levels above the sea. Clouds and 
mists being as wayward as they are, the climatic intensity of light 
in different places must be measured by actual observations. 
In 1862 Bunsen and Roscoe worked out a method of deter¬ 
mining the intensity of light, by noting the time required for the 
•darkening to a standard grey tint of paper impregnated with silver 
salts. Roscoe subsequently made a number of observations under 
different conditions and in different parts of the world by this 
method, and laid the foundation of what is known to meteorologists 
as “ Photo-chemical Climate ” ; which is so-called because silver 
salts only measure the intensity of the chemical rays at the blue 
end of the spectrum. For a first treatment of the subject these 
rays may give a proportional measure of the total light. 1 
In 1877 Hartig, in a short note, was the first to suggest the 
desirability of actual photometric studies in relation to vegetation, 
especially of the light prevailing in the depths of forests. His idea 
was never practically developed. 
In 1893 Wiesner worked out a convenient technique for 
thus measuring illumination, and proceeded to investigate the 
distribution of vegetation in association with it. An arbitrary unit 
of light-intensity was adopted, to wit, that light which darkens the 
photographic paper to the standard grey tone in one second. 
In the middle of the most brilliant summer days in Vienna the 
light-intensity may rise to P5 on this standard. The maximum 
intensity of total light ( i.e., direct sun plus diffuse light) varies from 
place to place with the height above the sea, i.e., with the thinness 
of the absorbing atmosphere. In Europe at 384 metres elevation, 
maxima of P7 to P9 may occur, while on a few chance days at 
Yellowstone Park (3,000 metres elevation) maxima above 2-0 were 
observed with the sun at 52°. In Buitenzorg with mid-day Sun 
1 Professor Wiesner describes the preparation of a new sensitive 
paper—Rhodamin B. paper—which is acted on by all spectral 
rays, especially the yellow. 
