WALTER E. GARREY . 121 



vertical surface or rest with the body axis in a vertical line. When 

 one eye has been blackened, they no longer walk vertically but veer 

 off at an angle toward the unblackened eye. There is an algebraic 

 summing of the tendency to follow the vertical path and to circle 

 toward the good eye which results in the oblique path. When at 

 rest, the body axis preserves this obhquity. Several butterflies with 

 one eye blackened were put upon an opaque, gray, vertical screen, 

 so placed in the laboratory that one side received the direct Hght from 

 the window while the other was shaded. On the shaded side the 

 angular deviation of the path was 15° from the vertical. When the 

 screen was reversed, the brighter illumination from the windows 

 caused a deviation to a path 45° from the vertical, and when placed 

 in the sunlight, the average path in which they walked was 75° from 

 the vertical. In the latter instance the circling tendency was often 

 noted, and the butterflies' path might deviate to the horizontal line; 

 they then completed a sharp turn downward toward the good eye and 

 swung completely around again, starting the vertical ascent, but at 

 once began to swerve off at an angle. If the butterflies are placed 

 upon a screen, the upper half of which is in the sunlight, the lower 

 half shaded, one is surprised at the prompt increase in the angular 

 deviation of the path as they cross the line from the shade into the 

 sunlight. The flies, Tahanus and Eristalis, were unable to ascend a 

 vertical screen when it was directly illuminated by bright window 

 light but move in circles on it; in dim light they ascend obliquely. 



Experiments on a Cylinder or Spindle. — Instead of a plane, vertical 

 surface an upright cylinder or stick may be used. The position 

 assumed by Proctacanthus (Fig. 8) is oblique to the vertical and 

 shows the typical flexion and extension of the legs due to blackening 

 one eye. When the spindle is placed in the center of the illumination 

 chamber described above, or when light is reflected uniformly from all 

 sides, the effects of variations in the intensity of illumination may be 

 determined. All helio tropic insects, with one eye blackened, ascend 

 such cylinders obliquely (Fig. 8), and the forced motion carries them 

 up in a uniform spiral path. Instead of measuring the angular ascent, 

 as on the plane, vertical screen, relative effects are determined by 

 counting the numbers of turns required to ascend a given distance. 

 The arrangement constitutes a crude photometer, for the brighter 



