﻿Brightness of Intermittent Illumination. 



647 



determined by a revolution counter working on the shaft to 

 which the disk was attached. 



Equal portions of sectors were cut out of the disk at even 

 distances round a ring, forming a number of equal apertures, 

 and the same number of equal shutters; the apertures were 

 not necessarily equal to the shutters in size. The outer 

 diameter of the ring was 40*3 cm. and the inner 24*3 cm. 

 The disk was placed in such position that as it revolved, the 

 lamp was alternately completely open to or completely 

 screened from the photometer. 



The fraction of the light cut off when the disk revolves is 

 the ratio of the size of a shutter to the size of a shutter and 

 aperture together. Since the lamp itself was not a line of 

 light, the times of complete darkness or full brightness were 

 less than that taken for a shutter or aperture to pass 

 respectively. 



The time curve of the flux of light on the photometer for 

 a disk with equal apertures and shutters is of the shape 

 shown in the diagram. 



o 



Tim 3 



AB is the time taken for one edge of a shutter to pass 

 from one side of the light to the other, and so completely 

 obscure it; CD is the time taken for one edge of an aperture 

 to pass from one side of the light to the other, and so com- 

 pletely open it. It was easily determined by measuring 

 what fraction of an aperture was occupied by the luminous 

 filament of the lamp. 



The time taken for this fraction to pass in front of the 

 lamp was subtracted from the time taken for a shutter to 



