18D6.] of Moonlight and Starlight compared with Candle. 323 



Plotting these curves as before, it is found that their distance apart 

 is 3*5, that is, the light of the stars is 2 3,5 = 11*2 times more photo- 

 graphic than the candle at 10 feet off. As the exposure was 90 

 times more prolonged in the first case, this means that the light is 

 only ll - 2/90 or 0*125 that of a candle at 10 feet, that is, it is equal 

 to one standard candle at 25" 7 feet distance, or that it is equal to 

 0-001515 candle (S.C.) at 1 foot. 



January 29, 1895. 



The exposure to starlight lasted from 10.15 to 10.45, or 30 minutes. The 

 exposure to candlelight was for 60 seconds at a distance of 7 feet. 



] 



Intensity of light 

 through the 



graduated scale, in 

 powers of 2. 



Transparencies of the photographed 

 scale due to 



Starlight. 



Candlelight. 



1-45 



99 -0 



99 -0 



1-75 



98-0 



98 -0 



2-20 



94-5 



95-0 



2-70 



82-5 



83-0 



3-3 



70-5 



71 -0 



4-0 



55-5 



55 -5 



4-8 



34-5 



35 



5'8 



18-5 



18-0 



6 3 



13-7 



14-0 



6-55 



11 -5 



11-5 



These two curves are practically identical, hence we may take it 

 that on this night exposure to starlight for 30 minutes was equal to 

 exposure to the candle for 1 minute at a distance of 7 feet, that is, 

 starlight is equal to one S.C. at 34*8 feet, or to 0*000825 candle (S.C) 

 at 1 foot. The variation may be due to two causes — (1) the state of 

 the atmosphere, or (2) to the difference in heavens ; the first is most 

 probable. It may be interesting to know that photographic trans- 

 parencies from negatives were made by contact during these expo- 

 sures of the scale. In all cases there was evidence of considerable 

 over-exposure in the starlight. 



It has been stated that in order to ascertain that the candle was 

 giving the normal illumination for the observations by moonlight, 

 comparisons of its light were made with a standard amyl acetate 

 lamp. This comparison was also made on these occasions. 



We may now compare the light of the full moon with that of the . 

 stars. Fig. 3 gives a map of the stars with, the planets Jupiter, 

 Mars, and ISTeptune shown in position for the last exposure. The 



