1896.] of Moonlight and Starlight compared ivith Candle. 321 



10.5 p.m., February 9, 1895. 

 Plate exposed to moonlight for 60 seconds through the graduated opacity 

 scale, another part of the same plate was exposed to the light of a 

 paraffin candle for 60 seconds at 7 feet distance. 



Intensity of light 

 transmitted through 

 the scale of opacity, 

 in powers of 2. 



Transparencies of photographed 

 scale due to 



Moonlight. 



Candlelight. 



1-45 



48-0 



100-0 



1-75 



42 



99-0 



2-20 



31 



96-0 



2-70 



24-0 



91 -0 



3'30 



16-5 



75 -0 



4-00 



12-7 



62 



4'80 



7-9 



42 



5'80 



5-4 



23-5 



6-30 



4-0 



17-0 



6*55 



3-8 



14-5 



Plotting these curves as before, we find that they are 3*1 apart ; 

 that is, that moonlight is 2 3 * 1 = 10 '7 times more intense than the 

 candle at 7 feet. Moonlight was therefore equal to 1 S.C. at 194 feer-, 

 or was equal to 0*266 S.C. at 1 foot. We thus find that the moon at 

 the fall is equal to 0*266 S.C. in its action on a bromide plate. In 

 the paper already referred to it was shown that when measuring 

 different intensities on a plate, the integral increase in effect of white 

 light increases in the same ratio as does the intensity of a part of the 

 spectrum whose wave-length is at the maximum part of the curve 

 of sensitiveness. With these plates the point of maximum sensitive- 

 ness is about X 4450. 



Taking Zollner's visual measure of moonlight, he found it to be 

 about 0*012 candle at 1 foot. Sunlight, reduced in brightness to 

 visual equality, I have found on a bright, clear day in summer, 

 near midday, to have a photographic value 28*5 times greater than 

 that of an amyl acetate lamp flame, and that a standard candle of 

 equal visual intensity has a photographic intensity of 1*1 times that 

 of an amyl acetate flame. This makes sunlight to have a photo- 

 graphic intensity, the. visual intensities being equal, of very nearly 

 26 times that of a standard candle. As moonlight is reflected sun- 

 light, it may be presumed that the two have the same quality. If 

 this be so, we should arrive at moonlight, being 0*01 visually of a 

 standard candle, the moon being at the full, a value which is near 

 that given above. This mode of comparison must necessarily be 

 only approximate, on account of the variable nature of sunlight, and 

 therefore of moonlight. 



