﻿906 Mr. R. A. Mallet on the Failure of 



an ordinary single metal dark slide, so arranged that it could 

 be slid across the slit and stopped in any of several positions 

 by means of a small spring loaded plunger. The wooden 

 board was fixed to the bench in an upright position, and a 

 well-fitting wooden shutter arranged to slide across the front 

 of the slit. This shutter was worked by hand. The source 

 of light was a 36-watt 12-volt gas-filied motor headlamp bulb, 

 connected in parallel with a voltmeter to a 12-volt accumu- 

 lator batf-ery. This battery was connected through a suitable 

 resistance to the town mains so as to form a charging circuit. 

 In the course of three months the voltmeter reading fell by 

 0*2 volt, so that the drop during one experiment was 

 negligible. The lamp was enclosed in a light-tight box with 

 a square hole cut in the front, with the object of removing 

 irregularities due to stray reflexion from the walls of the 

 room. The light was used at a constant distance of about 

 one metre from the plate, at which distance even illumination 

 over the whole of the slit was obtained. All exposures were 

 timed by a stop-watch, the error in moving the shutter being 

 of the order of 1/10 th of a second, which may be neglected, 

 in view of the length of the exposures used. 



It is evident that with an arrangement of this kind, if a 

 plate be exposed behind the wedge, a line will be photo- 

 graphed on it, the length of which will he a function of 

 the time of exposure and of the intensity of the incident 

 light. Furthermore, if two lines are photographed side by 

 side and the time of exposure varied while the intensity is 

 kept constant, then 



I e-^T P = I 1 6-^T/ 7 



a i, p(to—h) 



and hence p=i ff i nr> 



r logTo-loglx 



where p is the wedge constant, T and T x the times of 

 exposure, t and ^ the lengths of the lines. 



Determination of the Wedge Constant. 

 The wedge constant was determined for the light used by 

 exposing Paget " Half-Tone" plates behind it to light of two 

 different intensities, the time of exposure being kept constant. 

 The intensities were in the ratio 1 : 25, the variation being 

 obtained by altering the distance between the light and the 

 plate from one metre to five. Then, since TqzsTj, 

 I 6-p^ = 1 i€ -p<i j 



and hence p= * ° - — -• 



in — 1\ 



