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BOTANY: A. M. HURD 
a direct- vision spectroscope with a wave length scale attached. These screens 
were then fitted as windows in the ends of boxes painted black on the inside. 
Culture dishes were made by cementing together microscope slides, so that 
the light entering the boxes through the screens fell on a flat side of the dish 
and thus entered the water normally with a minimum loss from reflection and 
refraction. 
The electric arc was used as the source of Hght wherever possible because it 
gives all the desired Vv^ave lengths, with the result that the whole set of screens 
could be used in the same exposure, insuring for all the boxes identical condi- 
tions of temperature and duration of illumination. 
The spectroscopic analysis of the light transmitted by the screens gave the 
exact wave lengths which would act on the cultures behind them. The next 
step was to devise a means of making the intensities of the lights acting in 
each box equal so as to eliminate that most important, and hitherto largely 
overlooked, factor in light reactions. This done, variations in results obtained 
behind different screens might safely be attributed to differences in the quality 
of the stimulus. There have been several methods devised by which the 
relative intensities of monochromatic lights can be measured and made 
equal.i'^'^'^-' But all of these involve special apparatus not available for 
use in this investigation; so a simpler method was devised whereby the rela- 
tive intensities of the lights transmitted by the Wratten filter-screens were 
measured by means of a thermopile and galvanometer and made equal by 
varying the distances of the dark boxes from the electric arc such that at 
these distances the deflections of the galvanometer, when the thermopile was 
exposed in turn to the light behind each filter screen, were equal. This dis- 
tance was also measured with a piece of white glass as a filter screen which 
represented the removal of the control culture from the arc. The instru- 
ments used in this energy calibration were a Hilger thermopile and a d'Arsonval 
galvanometer. 
It seems necessary on account of the questions which have been raised 
during the course of this work, to state here that the thermopile is equally 
sensitive to the energy of the red and of the violet ends of the spectrum, and 
is, therefore, an accurate measure of the total amount of light acting behind 
each color screen. The difference between heat and light is only a matter of 
wave length. The thermopile measures light in terms of the electric current 
produced by the difference in temperature of the exposed and unexposed 
junctions; but it does so by virtue of the fact that the energy of whatever 
vibrations fall upon it, be they long and therefore heating in their physiologi- 
cal effect, or short and therefore perceived as light, is converted into heat 
energy upon being absorbed by the exposed junction of the thermopile. In 
other words, the light of the blue end of the spectrum produces an electro- 
motive force much less than that of the infra-red but no less measurable. 
Once these distances from the arc, at which the intensities of the light in 
each box are equal, are determined, the quantity of light energy can be varied 
