QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 247 



The consistency with which the deflection remained constant 

 at various intensities of a fixed ratio made it seem unnecessary 

 to run the whole series of intensities to estabhsh the last three 

 points on the curve, so the three-glower intensity alone was 

 used at 66|, 83| and 100 per cent difference. A difference of 

 100 per cent meant that one of the side beams should act against 

 darkness, which was exactly the condition under which the 

 preliminary trials were made. Accordingly, to establish this 

 point, 200 of the test trails were measured. The deflection 

 under these conditions should, theoretically, be 90°. But this 

 is not the case experimentally, largely because of the time factor 

 involved in orientation. The new orientation, even though it 

 may be eventually a perfect alignment with the rays of light, is 

 seldom instantaneously accomplished ; and as locomotion is con- 

 tinued while the larva is orienting, the angle, if measured from the 

 point where the light is turned on, is less than 90°. Figure 2 

 shows the varying angles in which the trails eventually come to 

 orientation. But the time factor is of importance only when 

 the relative difference is very great, and the amount of turning to 

 be accomplished before orientation is attained is large. To 

 ascertain its effect in the 100 per cent set, trails were measured 

 in two ways: first, exactly as all the other trails were measured 

 (fig. 6, p. 234) ; and again by measuring the final angle of orienta- 

 tion. Take, for example, the trails of figure 2, a, b, c; according 

 to the latter method of measurement, all of them eventually 

 attain a 90-degree angle to the line of starting; but according to 

 the former method, the time taken in orienting lessens the angle 

 measured. The deflection at 100 per cent, measured as the 

 smaller angles were measured was 77.57 degrees; the deflection 

 of the same trails measured by their final orientation was 83.78°.'' 

 The difference of 6.19° between the sets represents the time factor 

 in a change of orientation through 90°. The results of the first 



* The measurements of the "final angle of orientation" were made at the point 

 where the trails crossed the scale of the protractor of 7.6 cm. radius used in the 

 other measurements. Had the measurements been made after the larva had been 

 allowed to crawl through a longer distance, the determination for this set would 

 have been nearer 90°. 



