158 THE AMERICAN NATURALIST. [VoL. XXXIX. 
gradually came to rest in the darker portions of a shallow cylin- 
drical glass vessel placed before a window. The large majority 
of the flies resting thus on the vertical sides of the vessel had 
their long axes parallel to the light rays, and their heads turned 
away from the source of light. Parker (:03) has pointed out that 
the mourning-cloak butterfly, coming to rest in bright sunlight, 
also places itself with the head away from the sun. The posi- 
tion assumed by the flies under these conditions might have led 
to the supposition that they had become negatively phototropic 
through continued exposure to an intensity of light higher than 
that to which they were accustomed. Such reversals have been 
observed for barnacle larvae by Groom und Loeb (’90), for 
Hydra by Wilson (91), for Polygordius larvae by Loeb ('93), 
for Limax by Frandsen (:or), for copepods by Parker (:02) 
and for earthworms by Adams (:03). However, it was found 
that upon slightly turning the vessel on the table nearly every . 
fly flew instantly toward the window, thus giving marked evi- - 
dence of positive phototropism. 
The experiments about to be described were undertaken with 
a view to obtaining some definite quantitative statements as to 
the influences of light and gravity on the movements of the 
organisms; and most particularly was it desired to test the 
responses of the flies to a range of high intensities of light. 
For valuable advice and aid in devising the apparatus and con- 
ducting the experiments the writer is indebted to Professor 
G. H. Parker, under whose supervision the work was carried on. 
Apparatus — In all of the operations the flies were confined 
in a cylindrical glass vessel measuring 15 cm. in length and 5 
cm. in diameter, and closed at both ends by glass caps. Around 
the outside of the vessel were placed five small rubber bands at 
equal distances apart, the cylinder being thus divided into six 
sections each 2.5 cm. long. This subdivision of the vessel by 
‚surface markings enabled the observer to record the position of 
the contained fly at any moment with considerable precision. 
In some of the experiments a dark box of convenient size was 
used, a sectional plan of which is given in the accompanying fig- 
ure. This box was lined throughout with black cloth. In the 
center of each end wall was fixed an incandescent electric-light 
