218 Mr. Dyke on the Flux of Light from the 



The intensity of illumination of the screen is then pro- 

 portional to the mean spherical candle-power. As previously 

 arranged for testing- incandescent lamps, the photometer 

 measured directly the ratio of the mean spherical candle- 

 power to the mean horizontal candle-power. When dealing 

 with arc-lamps, however, this ratio is not of much practical 

 value, so a small alteration was made to adapt the instrument 

 to read the absolute value of the mean spherical candle- 

 power. This was effected by replacing the horizontal sliding 

 pair of mirrors by a standard incandescent lamp on a sliding 

 support. 



This alteration will of course alter the constant of the 

 instrument. 



The formula given before was 



M.S.C.P. _ 5-09 

 M.H.C.P. ~ d l 



where d is the distance in feet from source to screen by way 

 of the sliding pair of mirrors. 



Now the sliding pair of mirrors reflected 70*5 per cent, of 

 the light incident on them ; hence it is obvious that if these 

 mirrors are replaced by a lamp of known candle-power C.P. 

 we have 



M.S.CJ\ = ^x~- 



• t Oo a 2 



Hence the formula becomes 



M.S.C.P. = ^ 2 xC.P. of balancing-lamp, 



where d is the distance of balancing-lamp from photometer- 

 screen in feet. 



As this result is obtained somewhat indirectly the arc 

 was replaced by an incandescent lamp, and the polar diagram 

 constructed by opening the slides in front of the mirrors one 

 at a time and making a photometric measurement for each. 



The Rousseau diagram was then drawn, and the mean 

 spherical candle-power deduced and found to be 22*19. All 

 the mirror-slides were then opened and a balance obtained 

 at 15*95 inches, the candle-power of the balancing-lamp 

 being 5*51. 



Hence, applying the formula given above, 



,',*., , 22-49 / 15-95 V 



constant ot instrument = „ ,, x | , rt I 



5*51 \ 12 / 



= 7-21. 

 This check-experiment being considered satisfactory, the 

 value 7*22 was taken as the constant of the instrument. 



