The light of the night sky: its intensity variations when analysed by colour filters.—II

The present paper describes a further instalment of the work initiated in ‘Roy. Soc. Proc.,’ A, vol. 106, p. 117, 1924. Although the ultimate outcome of the whole investigation is far from being apparent as yet, some of the methods have been improved, and data accumulated which make it possible to be more definite on certain points. Since the scheme of observation has been in the main the work of one individual, both in conception and execution, it is important to place the results on record from time to time, to prevent any risk of their being lost. It will be remembered that the fight of the night sky was passed through a colour filter, and the intensity matched against a permanent self-luminous (radioactive) source, using neutral glasses of known opacity to reduce the two to equality. The filters used were three in number :— (а) Wratten 23A, orange, not transmitting the light of the green aurora line λ 5578.