256 Captain Abney and Major-Gen eral Festing. [Dec. 8. 



Mr. Preece's table is as follows ; the last column is derived from the 

 parabolic formula using C 2 — s instead of W—m — a*/y. 



Table V. 



Distance of source 

 of* light from illu- 

 minated surface. 



Equivalent 

 degree of 

 illumination. 



Current 

 in 

 lamp. 



C 6 x 15-994. 



Parabolic 

 formula. 



-50 feet 



64 -000 



1-260 



64 -000 



64-00 



0-75 „ 



28-445 



1-100 



28 -335 



32-83 



1-00 „ 



16 -000 



0-959 



12 -442 



16-00 



2 '00 „ 



4-000 



0-790 



3-888 



4*41 



3*00 „ 



1-778 



0-690 



1-726 



1-78 



4 -00 „ 



1-000 



0-651 



1-217 



1-00 



It having been shown that the parabolic formula applies to visual 

 measures of an incandescence light, it appeared that the same ought to 

 hold good for the total light which is photographically active. These 

 rays may be taken to lie between the blue and the extreme ultra- 



Fig. 4. 



