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



March 19, 1896. 



Sir JOSEPH LISTER, Bart., President, in the Chair. 



A List of the Presents received was laid on the table, and thanks 

 ordered for them, 



The following Papers were read : — 



I. a The Photographic Values of Moonlight and Starlight 

 compared with the Light of a Standard Candle." By 

 Captain W. de W. Abney, C.B., R.E., F.R.S. Received 

 March 3, 1896. 



Estimations of the visnal values of moonlight and starlight by 

 several observers are extant, bat, as might be expected, they vary 

 considerably from each other, more particularly when they are 

 referred to such a variable standard as sunlight. For instance, the 

 light of the moon is said by Wollaston to be roughly -g-oWoo °^ 

 that of the sun ; by Bouguer, to be 3~ooVoo- Probably Zollner's esti- 

 mate is a fair one, he taking it as -gTsVoo' * na * °^ sunlight. As far 

 as I am aware, the photographic values of moonlight and star- 

 light have not been recorded with any great precision, and I now 

 offer some determinations which I have made under favourable 

 circumstances, and referred them to the British standard candle, 

 which, though occasionally showing variation in the light it emits, 

 yet can be utilised when care is taken to check the results by a 

 reference to a standard such as a Siemens amyl acetate lamp, which 

 is extremely constant. My first recent attempt at measuring the 

 value of moonlight was made at Chamounix in the early part of 

 January, 1895. At that time the sky was cloudless, but on some 

 few occasions there was a very slight mist in the valley which pre- 

 cluded any great degree of accuracy from being attained on those 

 nights, and further, as I was obliged to leave for England before 

 full moon, the observations I made there are necessarily incomplete 

 so far as the object I had in view is concerned. I have thought 

 it worth while to give two examples of these observations, how- 

 ever, to show how necessary it is that both sky and atmosphere 

 should be perfectly clear from any mist or haze. I may state that 

 when the observations were made there was about 45° (F.) of frost. 

 The method of observation that was adopted was as follows: — A 

 scale of graduated opacity was prepared on a photographic plate by 



