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of the exposure of the light; the plates, amply long to begin with, are shortened with tape at their non-proximal ends and the speed of the disc is varied also by shifting the weights C and D.

Another source of error which might prevent the accurate control of the light inertias and also the brightness of the light would be the variations in the voltage of what is supposed to be a 125-v a.c. circuit. We have planned to circumvent this possible source of error by placing a rheostat and a voltmeter in circuit with our lights, cutting down the voltage to 110, and then by keeping the voltage at 110 by the rheostat for each experimentation. We reckon that the frosted windows, through which the light must enter the tachistoscope, and the system of reflecting mirrors inside it, with the interior painting, act somewhat as filters and hence make negligible more minute variations of brightness other than those which would arise from daily variations in voltage.

A final source of error in the use of our apparatus might conceivably arise from the untrue rotation of the disc; i.e., from lateral vibrations as the disc turns on its axle, resulting in the failure of the contact-makers to function here or there on the metal plates or in a non-synchronizing of the breakmake between the two plates G and H. We believe that three features of the apparatus prevent errors of this sort: (1) the pressure from opposite sides of the disc is kept constant and in force by springs which hold the contact-makers against the metal-plates. (2) A ballast, or fly-weight, is situated at 0, to counteract the pressure of the contacts L and M. And (3) the superfluous weight on the short lever-arm of E-F functions somewhat as a ballast. The fine hand-workmanship of the axle and bearings reduces to a minimum the lost motion.

To summarize: we believe that the vertical type of Dodge mirror-tachistoscope has virtues not possessed by the Whipple tachistoscope; nothing moves before the eyes, light is more equally distributed over the exposure-field, and a non-moving pre-fixation field (equal in brightness to the main field) is afforded. Our timing apparatus has these virtues; time can be controlled as accurately as, if not more accurately than,

with the Whipple timing-disc; it serves, with the Dodge apparatus, to present no moving object in the fixation-field; it serves to equate in both brightness and length of exposure the pre-fixation field with the main field; and, lastly, our timing device rests on one table (if necessary in an adjoining room) and the tachistoscope on another, so that there is no jarring of the apparatus during exposure.

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HOWARD C. WARREN, PRINCETON UNIVERSITY (Review) JOHN B. WATSON, 244 MADISON AVE., New York (Review) SHEPHERD I. FRANZ, UNIV. OF CALIF., So. BR. (Monographs) MADISON BENTLEY, UNIVERSITY OF ILLINOIS (J. of Exp. Psych.) S. W. FERNBERGER, UNIVERSITY OF PENNSYLVANIA (Bulletin) WALTER S. HUNTER, CLARK UNIVERSITY (Index)

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