162 THE AMERICAN NATURALIST. [Vor. XXXIX. 
dark box, with an incandescent electric-light bulb at either end. 
The lower light only was at first turned on, and the fly thus 
attracted into the lower section. Then the window of the box 
was closed, the light turned out, and the fly left for one minute 
in darkness. At the end of this period both the upper and lower 
lights were turned on simultaneously, and at the expiration of 
another minute the window was opened and the position of the 
fly noted. Five readings were taken in this manner for each of 
six flies, three males and three females. When the results were 
combined (see Table II) it was seen that, out of a total of 30 
readings, the flies had been detected 19 times in the upper half 
of the cylinder, no less than 16 of these observations being for 
the topmost section. In 11 cases the flies had remained below 
the middle of the cylinder. 
Table II. 
Distribution of six flies in a vertically placed glass cylinder after one 
minutes exposure to equal illumination above and below: the fies at the 
outset occupied the bottom section of the vessel, No. 6. 




Sections of : 
cylinder. Number of trials. Totals. 
I 2 3 4 5 
I 2 2 5 4 3 16 
2 2 o o I o 3 
3 o o o o o o 
4 o o o I o I 
5 o I o o o I 
6 2 3 I o 3 9 






When this distribution is compared with that for flies left 
undisturbed in darkness for an equal length of time (Table I), 
it would appear that light acts as a stimulant to locomotion ; 
and the rather striking efficiency of light in this respect is 
attested by the comparatively large number of flies found in the 
uppermost section of the cylinder. 
Directive Effect of Light. — Experiments to demonstrate the 
directive effect of light of moderate intensity may be thought to 
