I. The photographic values of moonlight and starlight compared with the light of a standard candle

Abstract

Estimations of the visual values of moonlight and- starlight by several observers are extant, but, 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 1/800000 of that of the sun; by Bouguer, to be 1/300000. Probably Zöllner’s estimate is a fair one, he taking it as 1/618000 that of sunlight. As far as I am aware, the photographic values of moonlight and starlight 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.