^BOTANY: A. M. HURD 
203 
by multiplying or dividing all the distances by the same multiple and the in- 
tensities in all will still remain equal to each other. By means of a photom- 
eter the actual amount of light acting in each box can then be determined 
by measuring the intensity in candle meters behind the white glass control 
at the proper distance from the arc. Then from the law of inverse squares, 
viz., that the intensity of Hght per unit surface varies inversely as the square 
of the distance from the source, the absolute intensity at any distance from 
the arc can be computed. So a Sharpe-Millar photometer was used to 
measure the intensity of the naked arc at the distance of the white light con- 
trol. But it was then necessary to correct the measurements so obtained 
for the absorption of light by the glass of the filters. This so-called absorp- 
tion coefficient was obtained by measuring with a Lummer Brodhum photom- 
eter the intensity of a light both with and without a screen of clear glass 
equal in thickness to that of the filters. It was found that glass 1.5 mm. 
thick absorbed 12% of the light falling upon it. Therefore to obtain the in- 
tensity of the light actually entering each dark box, it was necessary to take 
88% of the reading given by the photometric measurement of the unscreened 
arc at the previously determined distance of the white light control. 
The following table fists the colored screens used with the wave lengths 
they transmitted and the distances from the arc at which they were placed to 
make the intensity of light behind each equal to 1800 meter candles. The 
lack of agreement between these values and the energy curve of the spectrum 
is due to the individual absorption of the filters and also to the fact that they 
do not all transmit the same number of wave lengths. 
Table showing distances at which the intensities of light from an electric arc transmitted by 
Wratten light filters are equal 
FILTER NUMBER 
WAVE-LENGTHS IN 
ANGSTROMS 
COLOR 
DISTANCE FROM 
LIGHT 
INTENSITY IN 
METER CANDLES 
70 
6600-7000 
Red 
cm. 
320 
1800 
71 
6200-6800 
Red 
275 
1800 
72 
5900-6200 
Orange 
230 
1800 
73 
5600-5900 
Yellow 
250 
1800 
74 
5200-5600 
Green 
280 
1800 
75 
4700-5200 
Blue 
250 
1800 
76 
4000-4700 
Violet 
250 
1800 
Control 
4000-7000 
White 
340 
1800 
To obtain the spores of Fucus injlatus for the experiments, the fruiting 
plants were collected at low tide, kept over night in damp newspapers, and 
the next morning were dried slightly by exposing them to the air for about 
half an hour. Then when the fruiting tips were submerged in sea water in 
the culture dishes, large numbers of eggs and sperms were extruded end 
settled to the bottom of the dish. After removing the piece of plant the 
culture dish was placed in one of the little racks made to fit in the dark boxes 
