322 Capt. W. de W. Abney. Photographic Values [Mar. 19, 



Ifc may be asked whether there are good grounds for presuming 

 that the standard candle was emitting the same amount of light on 

 the occasions quoted. There can be little doubt that it was, as on 

 every plate was impressed a "time" scale by exposure to an amyl- 

 acetate lamp. A comparison of this with the scale given by exposure 

 to the candle showed that any variation was negligible. 



We next have to consider the photographic value of total starlight. 

 It appeared to me that the fairest way of arriving at it would be to 

 expose a plate through the graduated scale to the action of the light 

 in a horizontal position. This would practically be the illumination 

 of a piece of white paper laid in that position. The great point was 

 to get a clear horizon, or at all events an horizon which was not 

 obstructed to any great extent by any adjacent tree or building. The 

 top of South Kensington Museum answers to the necessary condi- 

 tions, as it is higher than any close building, except the Natural 

 History Museum towers, and these are at a considerable distance. 

 At Wimbledon also it was possible to expose a plate with advantage, 

 though the horizon was not absolutely clear from the spot which was 

 available. Exposures were made at both places, and the highest 

 value at South Kensington agreed with the highest value at Wimble- 

 don ; it is therefore unnecessary to give the latter. It should be noted 

 that it is a decided advantage to have more than one graduated scale 

 bearing identical opacities, as it enables simultaneous exposures to be 

 given at different localities. 



The results of two exposures will be given, as the others do not 

 differ widely from them. 



January 25 and 26, 1896. 



Exposure to shy from 11.30 P.M. on January 25 to 1 A.M. on January 26, 

 being I hour and 30 minutes. The exposure to candlelight was 

 for 60 seconds at 10 feet distance. 



Intensity of light 

 through the 



graduated scale in 

 powers of 2. 



Transparencies of photographed 

 scale due to 



Starlight. 



Candlelight. 



1 -45 



66 -5 



100-0 



1 75 



62-0 



100 -o 



2-20 



53 -0 



100-0 



2 70 



42-0 



100 -o 



3-30 



31-0 



94 



4-00 



23-0 



84-0 



4'80 



16 '5 



69 -0 



5 -80 



13 -0 



49 -5 



6-30 



11-0 



4] -0 



6-55 



io-o 



35-0 



