36 LIGHT IN RELATION TO TREE GROWTH. 
chiefly on the ability of the observer to determine the point at which 
total darkness begins. The chance for subjective error, owing to the 
variabilty of the human eye, is therefore very great. 
MEASUREMENT OF CHEMICAL LIGHT INTENSITY. 
Of all the physical methods of measuring light, the measurements 
of its chemical intensity have proved most satisfactory, and have 
contributed in a large measure to a better understanding of climate 
(Bunsen and Roscoe, 1862) and of the effect of light upon plant life. 
USE OF PHOTOGRAPHIC PAPER. 
The Austrian botanist, Prof. Julius Wiesner, was really the first 
who employed in a practical and thoroughly scientific manner the 
action of light upon silver chloride paper (photographic paper) for 
determining the lght requirements of plants. Before Wiesner, 
Theodore Hartig attempted to determine quantitatively the light 
requirements of forest trees by means of photographic paper, but he 
did not succeed in developing a thoroughly satisfactory instrument. 
Use of photographic paper for measuring light intensities was first 
made by Bunsen and Roscoe for climatological investigations, but 
Wiesner was the first to adapt this method to studies of plant life. 
The measurement of the chemical intensity of light is based on the 
well-known law formulated by Bunsen and Roscoe that products of 
light intensity and time of exposure correspond to darkenings of 
silver chloride paper of like sensitiveness. In other words, if two 
exposures produce the same tints on the same kind of paper in dif- 
ferent lengths of time, the light intensities are inversely propor- 
tional to the corresponding periods of exposure. Thus, if a certain 
shade is reached in light 7 by an exposure of ¢ seconds, and the same 
shade is reached in light 7, by an exposure of ¢, seconds, then 7: /,= 
t,:t, or Jt=I/t,. By varying, therefore, the time of exposure, it is 
possible to obtain identical shades at very different light intensities. 
“NORMAL SHADE” AND ‘‘ NORMAL PAPER.”’ 
In order to have a uniform and constant unit of measure of the 
light intensity a standard or “normal shade” and a standard or 
“normal paper” has been adopted. The “normal shade” adopted 
by Bunsen and Roscoe is prepared as follows: One thousand parts 
of chemically pure white zinc oxide are mixed with one part pure 
Jampblack heated to incandescence in the absence of air. The 
mixture is pulverized to a fine grayish powder. This powder is then 
spread, with the aid of a gelatinous solution, in an even coat on a 
piece of white paper or pasteboard, and produces a constant, uni- 
form, and somewhat grayish color. This color is the “normal 
shade ” to which al! other shades are compared. 
