LAURENS: MONOCHROMATIC LIGHTS. 261 



80 cm., thus giving ample space for work. The experiments that were 

 made to test the reactions to Hghts 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 onl}^ 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 A' (Fig. 1) being used to 



