248 BRADLEY M. PATTEN 



method of measurement are shown in the curve, as a solid line, 

 and those of the second as a dotted line (fig. 15). 



b. Application, With a series of measurements of this sort 

 established, not only the actual response at certain fixed intensity 

 differences is known, but the consistent progression of the values 

 of these determinations makes it possible to predict by reference 

 to the curve plotted from them the response that would be 

 evoked under any intermediate degree of stimulation. This 

 should be of practical value as a comparative physiological 

 measure of other experimental results. 



For example, the responses of the blowfly larva to colored lights 

 have been carefully worked out by Gross ('13, p. 475): "The 

 colored lights .... were obtained by cutting down the 

 spectrum by means of diaphragms of blackened cardboard with 

 narrow vertical slits of appropriate size." The lights used were 

 therefore practically monochromatic, while the intensity of the 

 beams was accurately measured by the radio-micrometer.^ Thus 

 the two greatest errors commonly present in colored light work 

 (i. e., the use of mixed light due to imperfect color screens, and 

 the inaccurate determination of the intensity) were eliminated 

 at the start. 



Gross subjected the larvae to opposed beams of light of different 

 wave lengths but of equal radiant energy, and recorded the de- 

 flection made toward one or the other of the lights. His figure 

 summarizing the reactions under opposed green and blue lights 

 of equal intensity is reproduced in figure 17, I have added to 

 his figure the labelling of the sectors in degrees, and the pair of 

 heavy lines which represent the average deflection. In making 

 his records, larvae were started in the green light and allowed 

 to crawl in the direction of its rays, along the 90-degree line of 

 figure 17, until they reached the center of the circle. Here the 

 blue light was turned on from the opposite side. The course 

 then taken by the larva under the two opposed lights was re- 

 corded by indicating the sector through which it emerged from 

 the circle. Figure 17 records in this way the points of emer- 



^ For description of method by which the colored lights were equalized by the 

 radiomicrometer, see Day, 1911. 



