184 Parr .— The Response of Pilobolus io Light. 
suspended mirror could be read. Small magnets placed near the galvano¬ 
meter served to adjust its sensitiveness. The wires connecting the galvano¬ 
meter to the thermopile were insulated and enclosed in a glass tube, which 
in turn was covered with tin-foil connected with the earth to prevent 
disturbances arising from slight variations of room temperature. 
Much difficulty was to be anticipated in the manipulation and adjust¬ 
ment of instruments of such delicacy and precision. Although set up 
on a basement floor of heavy cement, the closing of doors in adjoining 
rooms or walking in the passage-ways caused tremors that interfered with 
galvanometer readings. Street cars passing a block away could be detected 
by a change in the sensitivity of the galvanometer. For these reasons the 
determinations were made at night and a time chosen when weather 
conditions were quiet and when very few people were working in the 
building. 
The light sources used were a Nernst lamp of single glower type, 
obtained from the Westinghouse-Nernst Company, and a 200-watt nitrogen- 
filled tungsten Mazda made by the General Electric Company. Both were 
used on a no-volt alternating current from the University power plant. 
The voltage was regulated by a volt rectifier and was remarkably constant. 
Each lamp was adjusted and carefully centred for the lens and prism. The 
lamp once adjusted remained fixed throughout a series of tests. In the 
Nernst lamp the glower, being 1 mm. in diameter, served in the place of 
a narrow slit. A wide slit in front of the glower served to admit the direct 
light to the lens and to cut off all lateral radiation. The light projected on 
the prism from the nitrogen-filled tungsten lamp was passed through a slit 
approximately 5 mm. wide in front of the globe. 
The spectral region admitted to the experimental chamber was 
sufficiently large to cover the slit of the thermopile. Each region used was 
admitted through a separate slit (2*5 X 10 mm.) cut in the slides of a photo¬ 
graphic plate-holder. The frame for these holders was permanently attached 
to the base-of the apparatus and the slide corresponding to the region to be 
studied was inserted at the focus of the rays for this region. The thermo¬ 
pile, when measurements were in progress, was placed at 10 cm. from 
the slit. 
The standard of light energy used was a Hefner lamp manufactured 
by Max Kohl and tested in the German Reichsanstalt. The conditions 
prescribed for its use were strictly followed, and it was placed at 2 metres 
from the thermopile, the latter enclosed in the experimental chamber. The 
energy value of this Hefner lamp at 2 metres distant from the thermo¬ 
couple was determined in terms of the deflexion of the galvanometer. The 
energy of the light from any of the spectral regions from the different light 
sources was similarly read, and expressed in terms of the Hefner lamp 
reading. 
