1887.] Oil Sunlight Colours. 69 



the right-angled prism through the spectrum. The two stripes are 

 now apparently equally luminous, and I see the point of equality is 

 where the edge of the right-angled prism is in the green. Placing a 

 narrow cell filled with our turbid medium in front of the slit, I find 

 that the equality is disturbed, and I have to allow more of the yellow 

 to come into the patch formed by the blue end of the spectrum, and 

 consequently less of it in the red end. I again establish equality. 

 Placing a thicker cell in front, equality is again disturbed, and I have 

 to have less yellow still in the red half, and more in the blue half. I 

 now remove the cell, and the inequality of luminosity is still more 

 glaring. This shows, then, that the rays of maximum luminosity must 

 travel towards the red as the thickness of the turbid medium is 

 increased. 



The observations at 8000 feet, here recorded, were taken on 

 September 15 at noon, and of course in latitude 46"^ the sun could not 

 be overhead, but had to traverse what would be almost exactly 

 equivalent to the atmosphere at sea-level. It is much nearer the 

 calculated intensity for no atmosj)here intervening, than it is for one 

 atmosphere. The explanation of this is easy. The air is denser at 

 sea-level than at 8000 feet up, and the lower stratum is more likely 

 to hold small water particles or dust in suspension than is the higher. 



For, however small the particles may be, they will have a greater 

 tendency to sink in a rare air than in a denser one, and less water 

 vapour can be held per cubic foot. Looking, then, from my 

 laboratory at South Kensington, we have to look through a propor- 

 tionately larger quantity of suspended particles than we have at a 

 high altitude when the air thicknesses are the same ; and consequently 

 the absorption is proportionately greater at sea-level than at 8000 feet 

 high. This leads us to the fact that the real intensity of illumina- 

 tion of the difierent rays outside the atmosphere is greater than it is 

 calculated from observations near sea-level. Prof. Langley, in this 

 theatre, in a remarkable and interesting lecture, in which he de- 

 scribed his journey up Mount Whitney to about 12,000 feet, told us 

 that the sun was really blue outside our atmosphere, and at first blush 

 the amount of extra blue which he deduced to be present in it would, 

 he thought, make it so; but though he surmised the result from 

 experiments made with rotating disks of coloured paper, he did not, 

 I think, try the method of using pure colours, and consequently, I 

 believe, slightly exaggerated the blueness which would result. I 

 have taken Prof. Langley's calculations of the increase of intensity 

 for the difi'erent rays, which I may say do not quite agree with mine, 

 and I have prepared a mask which I can place in the spectrum giving 

 the different proportions of each ray as calculated by him, and this 

 when placed in front of the spectrum will show you that the real 

 colour of sunlight outside the atmosjohere, as calculated by Langley, 

 can scarcely be called bluish. Alongside I place a patch of light 

 which is very closely the colour of sunlight on a July day at noon 

 in England. This comparison will enable you to gauge the blueness, 



