1902.] 



Contributions to the Study of Flicker. 



317 



that S was sufficiently raised above the working bench to prevent any 

 rays of light from falling on the bench between S and D, and so 

 getting reflected on to the disc or about the room. By several screens 

 such as SS' the stray light was further confined, with so good a result 

 that, when it was necessary to bow one of the standard forks used to 

 determine the pitch of the note given by the twelve syren holes in the 

 rim of the disc, it was done chiefly by feeling, the forks being arranged 

 in the order of their pitch on a neighbouring table and generally in 

 charge of an assistant. Q in the figure is a small screen with two eye- 

 holes through which the observer looks at the disc, and which can be 

 arranged at any desired distance from D; some blackened screens, 



Fig. 2. 



CCC, receive any stray rays after they have passed D, so that the 

 room may be described as very nearly dark (when the air is not 

 unusually dusty, for of"Course any dust in the path of the rays scatters 

 .some light). The disc D is mounted on an electric motor, running 

 smoothly, and since some of the sources of light used were electrical, 

 it is important to state that the battery running the motor was alto- 

 gether separate and independent of that which supplied the source of 

 light. It is also necessary to give some account of the means by 

 which the powers of the illuminants employed were gauged, the 

 range of illuminations being unusually great. The photometer is 

 shown in fig. 1 ; ABC and EDO are two right-angled prisms, having 

 their faces BC and CD in one plane ; both these faces are finely and 

 evenly ground : light from the two sources is incident normally on the 



