50 
observer to express in millimeters the exact length of the long and 
short axes of the pupil. The device which proved to be most satis- 
factory was made for me by Messrs. Gaertner & Co., of Chicago. Their 
‘^simple comparator” was modified by the author to meet the needs 
Fig. 2. — Pupilometer. 2a, Reading microscope with, cross wires; 26, microme- 
ter slide (see fig. 3); 2c, adjustable substage (see fig. 4); 2d, support; 
2e, water bath (see fig. 1). Temperature regulators and light omitted for 
sake of clearness. 
of the present series of experiments. (See fig. 2.) The essential parts 
of the apparatus are, (1) a reading microscope with cross wires in the 
eyepiece (fig. 2a), (2) a micrometer slide (fig. 2b and fig. 3), (3) an 
