364 



Mr. H. E. Ives 



on a 



fig. 4 is exhibited the brightness of each glass due to illumin- 

 ation by a light source on one track alone, as the glasses are 

 rotated. The sum of the two is calculated in order to 

 facilitate moving the light sources, so that the mean brightness 

 of the two glasses may be maintained constant as their 

 relative values are changed by their rotation. 



Fiff. 4. 



C 

 4- 



-C 



.©■> 

 03 



Sum 







x 1 



15* 3(f 



45° 



60° 



75° 



Relative brightness of upper and lower glasses of mixture photometer 

 due to a single light source, for various angles of incidence of light 

 on lower glass. 



Using one light source, this device permits the comparison 

 of fields of all degrees of relative brightness, but of the same 

 colour. Using two light sources of different colour, but the 

 same intensity, the device makes possible the comparison of 

 fields of all hues intermediate between the two, formed by 

 their mixture. 



As a precautionary measure, the light transmitted by the 

 two opal glasses was tested for any difference in polarization 

 which would affect the setting of the polarization flicker 

 photometer, but nothing of the sort was found. 



4. Critical Speed Relations for various relative Brightnesses 

 of Two Compared Fields. 



(a) Mean Brightness Constant. 



In the second of the papers on the theory the following 

 equation is derived for the critical speed of disappearance of 



