194 WILLIAM BRODBECK HERMS 



amount of stimulation due to rolling the larva into place; under 

 such conditions it makes many randon movements, is clearly un- 

 directed, and soon comes to rest. 



The large number of indifferent reactions recorded for the 

 lowest intensity and total darkness is based on the behavior just 

 described; whereas the reactions recorded in the same column for 

 higher intensities may also be based on movement to right and 

 left, neither positive nor negative (commonly designated as zero 

 movements). 



5. Light graded in intensity 



Two methods of light grading were emploj^ed. The first was 

 by means of the light grader and was of non-directive nature 

 i.e., vertical light directed on a horizontal stage on which the 

 animal moved. A field of light was thus secured ranging from a 

 maximum of 70.69 CM. to within a given distance. The sec- 

 ond method, illustrated by fig. 11, gave a field of graded light 

 directive in its nature, but gradually increasing in intensity over a 

 given area in the direction of the rays. Such a field was produced 

 from the vertical filaments of a 66 cp. incandescent lamp (L) by 

 placing an opaque screen (C) in front of it in such a manner that 

 the light from the entire length of the filament could reach the 

 horizontal stage at a point {B), one metre distant from the upper- 

 most portion of the filament. On both sides of this point (B) 

 the light gradually diminished in intensity; toward the filament 

 it diminished because less and less of the filament was in range of the 

 stage, and away from the filaments it diminished because of the 

 radiation of the light. 



The behavior of the larvae in reference to the two methods will 

 be discussed separately: first, behavior under non-directive graded 

 light, and secondly, behavior toward directive graded light in the 

 above sense. 



A. Non-directive graded light. A field of light of this character 

 was produced by means of the light grader already mentioned. 

 Triangular diaphragms of various altitudes were used, the extreme 

 lengths producing fields of light 2 cm. broad and respectively 

 10 cm. and 2 cm. long. The intensity of the light in the strongest 



