318 Capt. W. de W. Abney. Photographic Values ' [Mar. 19, 



furnish a " time " scale on the same plate. Their visual illuminating 

 power was first of all compared, and at tbe distances where they 

 balanced one another, the exposure to the plate was given. 



The paraffin candle employed was found to be 1'085 of tbe standard 

 candle. The exposures were given to the plate at 10 feet and 

 9*6 feet respectively. 



The following table gives the results : — 



Exposure given, 



Standard candle 



Paraffin candle 



in seconds. 



at 9 ft. 6 in. 



at 10 ft. 



20 



79 -0 



71 



40 



51 -5 



46-0 



80 



32-5 



29 5 



160 



19 -0 



215 



When the curves are plotted, it will be found that they are at such 

 a distance apart that the light of the paraffin candle is photographic- 

 ally 1"21 times more luminous than that of the standard candle, 

 though visually they had the same illuminating value on the screen. 

 All results obtained with the paraffin candle have therefore to be 

 increased in that proportion to bring them into the values of standard 

 candles. 



7 P.M., January 4, 1895. 



Plate exposed to moonlight through the graduated opacity scale for 

 60 seconds ; another part to the light of a paraffin candle at 5 feet 

 for 60 seconds. 



(As before, the light fell normally on the surface of the plate.) 

 The moon had during exposure an altitude of about 50° ; the light 

 had, therefore, to traverse about 1*3 atmospheres. 



Intensity of light 

 transmitted, in 

 powers of 2. 



Transparencies of photographed 

 scale due to 



Moonlight. 



Candlelight. 



1-45 



93-5 



92 



1-75 



92-0 



87 



2*20 



87-0 



80 



2-70 



80-0 



70 



330 



69 -0 



60 



4-00 



56-0 



49 



4-80 



42-5 



35 



5 '80 



25 '5 



22 



6-30 



20-5 



16 



6 55 



19 



15 



