1896.] of Moonlight and Starlight compared with Candle. 317 



The accompanying figure is from a photograph of an exposure to 

 moonlight and candlelight through the scale of opacities, and of a 

 ■" time " scale made with an am yl acetate lamp. This will indicate 

 the method which was adopted better than a verbal explanation. 



I is the time scale, the exposures being made to an amyl acetate 

 lamp at a convenient distance. 



II is the exposure to candlelight through a graduated scale. 



III is exposure to moonlight through the same graduated scale. 



10.5 p.m., January 3, 1895. 



Plate exposed to moonlight for 90 seconds through the graduated opacity 

 scale ; another part to the light of a paraffin candle at 5 feet for 

 60 seconds. 



(In both cases the light fell normally on the plate.) The moon's 

 altitude during the exposure was only about 16° ; the light had, 

 therefore, to pass through 3'6 atmospheres. 



Intensity of light 

 transmitted through 

 the scale in powers 

 of 2. 



Transparencies of photographed 

 scale due to 



Moonlight. 



Candlelight. 



1-45 



100 



86 



1*75 



100 



82 



2*20 



100 



74 



2'7 



98 



65 



3 3 



92 



55 



4-0 



85 



42 



4'8 



71 



30 



5-8 



53 



20-5 



6 3 



43 



17 



6-55 



39 



16 



Diagram 2 shows these figures plotted graphically. The distance 

 apart of the two curves is 2*3, that is, the light of the moon on this 

 occasion was 2 2-3 = 4*95 times less photographically bright than that 

 of a candle at 5 feet distance, or was equal to one standard candle at 

 very nearly 10 feet, or closely 0'01 standard candle at 1 foot 

 distance. As the exposure to the candlelight was only two-thirds of 

 that to the moon, this result must be reduced accordingly. On this 

 occasion there was a slight ground haze; the low results are due 

 largely to the atmospheric absorption. 



The results have been given in standard candles (which we will 

 in future designate as S.C.). The value of the paraffin candle 

 employed was compared photographically by employing each to 



z 2 



