No. 459.] REACTIONS OF POMACE FLY. 159 
bulb (L.), which projected into the interior. A determination 
of the amount of light given off from the ends of these bulbs 
showed each to have a candle power of approximately 5. The 
lighting of both these bulbs could be controlled from the out- 
side by means of keys. A window let into one side of the box 
gave access to the interior when desired. This window could 

D 








indicate the division into six sections by rubber ; 
roken line surrounding V isa projection of the outline of the window of the dark 
box on the plane of this drawing. : x 
be closed by a tight-fitting shutter. When the dark box was 
used the glass cylinder (V.), containing the flies, was placed 
within, midway between the free ends of the two electric-light 
bulbs, its axis coinciding with the straight line connecting these. 
The distance from either end of the cylinder to the adjacent 
electric light filament was 7 cm. Between each bulb and the 
cylinder was inserted, as a heat screen, a glass vessel (W.), filled 
with water, and having flat, vertical sides 3 cm. apart. Both 
the glass cylinder and the heat screens rested on strips of wood 
painted black; and these supports could be so arranged that 
the box, with cylinder and heat screens in place, could be used 
in either a horizontal or a vertical position. 
Kinetic Effect of Mechanical Stimulation.— If in ordinary 
daylight a fly be placed in the glass cylinder, and this be held 
vertically, the fly will seek the top. If now the vessel be turned 
upside down the fly, finding itself at the bottom, will creep up- 
ward again to the top within considerably less, as a rule, than 
