After securing exact balance, the double V-shaped screen is kept fixed, and the leaf mounted in a rectangular
Fig. 93. Equal responses in opposite directions by alternate illumination of the two pairs of quadrants.
dark chamber closed except at the front, which carries a photographic shutter by which one pair of quadrants is exposed to light for a definite duration. The electric connections with the leaf are led to four binding screws; the petiole protruding from the box is dipped in an U-tube filled with water (cƒ. right-hand illustration of fig. 92).
The source of light is an arc or an incandescent lamp, placed inside a lantern, the condenser of which sends a parallel beam of light. A rectangular glass trough filled with alum solution is interposed in the path of light to absorb the heat rays. The duration of exposure is varied according to the sensitiveness of the specimen; the usual period of exposure is about 20 seconds.
