Illumination and the Measurement of Light. 275 



of light from 

 illuminated 

 surface. 



Equivalent 

 degree of 

 illumination. 



Current through 

 lamp required to 

 produce balance 

 of illumination. 

 (C.) 



C 6 x 15 -994. 



Feet.j 





Amperes . 





•50 



64 -000 



1-260 



64 -000 



•75 



28 -445 



1-100 



28 -335 



1 -oo 



16 -000 



•959 



12 -442 



2-00 



4-000 



•790 



3-888 



3-00 



1-778 



•690 



1-726 



4-00 



1-000 



•651 



1-217 



From these experiments it appears that the illuminating power of 

 the glow lamp increases in the ratio of the sixth power of the current. 

 Hence it is only necessary to read the current that gives the necessary 

 equivalent to determine the degree of illumination. 



The current in amperes thus gives, for the particular glow lamp 

 employed, a number whose sixth power expresses the amount of the 

 illumination measured. 



The apparatus works with ease and regularity, but it has this 

 difficulty — that it is dependent upon the constancy of the lamp, a 

 point not yet absolutely attained. The glass envelope of the lamp 

 becomes deadened by use ; the carbon fibre of the lamp gradually 

 becomes deteriorated ; and the vacuum sometimes fails. These 

 operations, however, are very slow ones, and it is only necessary 

 occasionally to compare either the light given by unit current with 

 that in the laboratory thrown upon a surface by a standard candle or 

 by one of Mr. Vernon Harcourt's pentane lamps. 



It is, however, certain from numerous experiments and practical 

 trials that the light produced by the incandescence of a given carbon 

 filament due to the passage of a given current is more easily repro- 

 duced from one time to another, and is more uniform than probably 

 any other artificial standard of light. 



