QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 229 



animal, the animal puts its median plane at right angles to the 

 line connecting the two sources of light." It follows, as a corol- 

 lary to this hypothesis that if the sources of light are unequal, a 

 negatively heliotropic animal would deflect from the perpendicular 

 toward the weaker light. This has already been demonstrated 

 by Mast ('11, p. 179) for the blowfly larva. 



By measuring in degrees the deflection toward the weaker light, 

 physical units of measurement may be applied to the physiologi- 

 cal phenomenon. And since with equal lights there is no deflec- 

 tion of the trail from the perpendicular to the line connecting 

 the sources, whatever deflection there may be with unequal lights 

 may be regarded as due to the effect of the difference between 

 the lights. The starting point for a series of experiments of this 

 sort would be at equality of the two beams of light. But it is 

 not enough to know merely that there is no marked deflection 

 when the lights are of equal intensity. It must be determined 

 how many degrees from the perpendicular the aggregate re- 

 sponse of a number of individuals falls. This necessitates not 

 only a method of measuring each trail, but also a method of com- 

 piling the data obtained by measuring many trails made by 

 different individuals. 



The way in which we have obtained our results may be seen 

 from a statement of (1) the method by which the trails were 

 made, (2) the method of measuring single trails, (3) the method 

 of tabulating the measurements. 



5. Making the trails 



The handling of the larvae in making the records is of sufficient 

 importance to be described in detail. At each end of the obser- 

 vation slate was a small wooden block, not shown in the dia- 

 gram (fig. 1), with a groove in it leading down to the level of the 

 slate. The blocks were located just in front of the orienting 

 lights, the grooves extending along the line connecting the two 

 lights (fig. 1, a and 6). The larva to be tested was gently rolled 

 from its box into the groove in front of the nearer orienting 

 light (fig. 1, a), with its head away from the light. All other 



