LAURENS: MONOCHROMATIC LIGHTS. 261 
80 cm., thus giving ample space for work. The experiments that were 
made to test the reactions to lights of different wave-lengths may be 
divided into two main sets; (1) Reactions to single monochromatic 
lights, and (2) Reactions to balanced pairs of monochromatic lights. 
In the first, the animal to be tested was exposed to a single light, which 
impinged upon its side at right angles to its long axis. In the second, 
the animal was placed midway between two lights of different, or of 
the same, wave-lengths, which impinged at right angles upon opposite 
sides of its body. A rather full description of the way in which the 
combined apparatus was used in making tests of the reactions will not 
be out of place. 
In preparing the apparatus for tests with single monochromatic 
lights, the following steps were taken: the four lights used (p. 259) were 
balanced, that is, made equal in the energy they contained, by adjust- 
ing the diaphragm openings (p. 259) one after another, till they gave 
equal deflections in a radiomicrometer. This was done only for the 
lights in generator A; for convenience these lights will be called the 
standard lights. From the measurements with the radiomicrometer, 
it was found that, in order to procure four colored lights that would 
be equal in the energy they contained, it would be convenient to 
use three forms of lamps, these lamps differing, however, only in the 
number of glowers employed in each. The lamp used for obtaining 
red light contained a single glower; that for yellow, two glowers; and 
that for the green and the blue, three glowers. A separate cardboard 
diaphragm, in the holder G, with a vertical slit of a different width 
and position for each light was also necessary. ‘These diaphragms 
were set in position accurately by means of a spectroscope, the neces- 
sary range for each light having been worked out experimentally. 
The lights from generator B were now adjusted so as to cover the same 
spectroscopic range as those from generator A. In this way the two 
lights were not only made equal spectroscopically, but they were also 
equal in intensity. This last fact was clearly demonstrated when a 
Lummer-Brodhun photometer was placed midway between two lights 
of the same wave-lengths proceeding from the two generators, the 
difference in intensity of the two lights being exceedingly small. 
Perhaps all this can be made clearer by a concrete case. Suppose, 
for example, that it was desired to test the reactions in blue light. 
The procedure was as follows: The 3-glower lamps were placed in both 
boxes and lighted, the appropriate diaphragms were next placed in 
position, and the wave-lengths of the light proceeding from generator A 
read on the spectroscope, the adjustment at X (Fig. 1) being used to 
