The ratio of the chemical action of the rays of light falling directly from the sun to the chemical action of the light diffused over the whole sky can be determined by means of an instrument, in which the shadow of a little ball is made to fall on a sensitive paper so as to intercept the direct rays of the sun, and allow it to be impressed by an action of the light diffused over sky alone; this compared with a similar paper, on which both the direct and indirect light has fallen, gives the ratio required. From this it appears, that the relative amount of chemically active light which comes directly from the sun, is very much less than the amount of his direct visible light. For while Professor Roscoe was making experiments at Manchester on the maximum effect of the chemical action of light, he found when the sun had an altitude of 20°, that of 100 chemical rays which fell on a piece of standard paper, only about 8 came from the direct light of the sun; while on the contrary, of 100 rays of visible light, 66 came directly from the sun, and only 40 from the light diffused over the whole sky, so that the diffused light is richer in chemical rays than the direct solar beam, ‘a startling result,’ but borne out by observations not only made at Manchester and in its vicinity, but at Kew, Heidelberg, and at Pará on the Amazon nearly under the equator. On account of the increasing rarity of the atmosphere, the greater the height above the level of the sea, the less the amount of diffused light and consequently of actinic power. Hence photographers have to expose their plates for a much longer time to the light on the snowy peaks of the Alps and other great heights than in England or at the level of the sea. During Mr. Glaisher’s tenth balloon ascent simultaneous observations were made at Greenwich Observatory and in the balloon, when at more than three miles above the surface of the earth, the standard paper exposed to the full rays of the sun was not as much coloured in half an hour as the corresponding paper at Greenwich in one minute. By a series of observations at Heidelberg, Kew, and Manchester, it has been proved that the very small relative chemical action of the sun’s direct light decreases rapidly with his altitude, and at these three places of observation, it has frequently happened when the sun’s altitude was very low, as at 12°, that his direct light made no impression on a sensitive paper. ‘The sun’s light had been robbed of its chemical power in passing through the air.’ This singular result is ascribed by Professor Roscoe to what he calls the opalescence of the atmosphere.