320 Capt. W. cle W. Abney. Photographic Values [Mar. 19, 



remarkably free from haze. The moon was full on February 9 at 

 • r >.23 p.m. An exposure was made on February 8 at 7.30 p.m. at 

 South Kensington, and another at 10.5 P.M. on February 9, near 

 Wimbledon, away from buildings of any importance. These two 

 localities were chosen to see if the country gave results differing- 

 from those obtained in town. At 7.30 on February 8 the altitude of 

 the moon was 35°, so that the light had 17 atmospheres to traverse. 

 At 10.5 p.m. in Wimbledon it had an altitude of 43°, so the light had 

 to traverse but 1'45 atmospheres. 



The results arrived at show that the night at Wimbledon on this 

 particular occasion was clearer than at Kensington, even allowing 

 for the absorption of the atmosphere. Tn my paper " On the Trans- 

 mission of Sunlight through the Earth's Atmosphere," it was shown 

 that the exponential coefficient of absorption in the clearest atmo- 

 sphere was 0340 for the rajs affecting the silver salt employed, and 

 adopting this it will be seen that there must have been a greater 

 haze in London than at Wimbledon. That this is not always the 

 case is shown that in measurements of starlight the two give identical 

 results on the same night. 



7.30 p.m., February 8, 1895. 



Plate exposed to moonlight for 60 seconds through the graduated opacity 

 scale, and another part of the same plate to a paraffin candle at 

 5 feet distance for 60 seconds. 



Intensity of the light 

 transmitted through 

 the scale in powers 

 of 2. 



Transparencies of the photographed 

 scale due to 



Moonlight. 



Candlelight. 



1-45 



93-0 



1C0-0 



175 



84 -0 



100 -o 



2-20 



74-0 



97-0 



2-70 



64-0 



88-0 



3'30 



49 -0 



74-0 



4-00 



35-0 



60-0 



4-80 



22 -5 



41 -5 



5-80 



13-2 



24-5 



6-30 



10 -o 



18-0 



6 "55 



9-0 



15 -0 



Plotting these curves, as shown in Diagram 2, we find that their 

 distance apart is l'l ; that is, the light of the moon is 2 1,1 — 

 2*4 times brighter than the candle at 5 feet. Moonlight was there- 

 fore equal to 2*9 standard candles at 5 feet, or equal to 1 S.C. at 

 2*94 feet; that is, moonlight was equal to 0*116 S.C. at 1 foot. 



