644 Mr. J. S. Dow on a Form of 



Paschen * has found that the electrons ejected by radium 

 bromide (that is to say, by radium C) possess different 

 velocities, but may be divided into two groups, the average 

 velocity of the electrons of one of the groups being greater 

 than that of the other group. These two groups of electrons 

 might possibly correspond to the above two principal groups 

 of rays. 



It gives me much pleasure to thank Prof. Thomson for his 

 inspiring interest and advice during these experiments. 



Cavendish Laboratory, Aug. 14, 1907. 



LX1IL A Form of Cosine Flicker Photometer. 

 By J. S. Dow, A.C.G.I., B.Sc. f 



[Plate XVI.] 



ri^HE illumination of the white surface employed in any 



photometer is equal to —^ — -, [where I equals the 



intensity of the source illuminating the surface, d the 

 distance of this source from the surface, and 6 the angle 

 between the rays of light striking the surface and a normal 

 to the surface]. 



Hence, when measuring the intensity of a source of light, 

 we may either vary " d," in which case we utilize the inverse 

 square law, or #, in which case the cosine law is utilized. 



While the inverse square law is almost invariably utilized 

 in photometric measurements, this method is inconvenient in 

 one respect. In order to vary "d" the photometer is usually 

 moved to and fro between the two sources of light to be 

 compared. The observer is therefore obliged to be continually 

 moving his head in order to follow the motion of the photo- 

 meter, and this is particularly distracting when the eye is applied 

 to a telescope. In order to avoid this necessity, many workers 

 prefer to keep the photometer stationary and to move one of 

 the sources of light. But in the case of gas-lamps and many 

 other sources of light, this method is obviously unsatisfactory, 

 and, even in the case of glow-lamps, is sometimes inconvenient. 



The utilization of the cosine law is advantageous in this 

 respect, for the photometer may then be kept stationary and 

 the illumination of the photometrical surfaces adjusted in the 

 photometer itself. The type of instrument about to be 

 described by the author, and shown in fig. 1 (PL XVI.), has this 

 advantage. Indeed, while it is desirable that such a photometer 



* Paschen, Ann. der Phys. xiv. p. 389 (1904). 



t Communicated by the Physical Society : read June 28, 1907. 



