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water cell will be less for the red star, Class M, than for the yellow star, Class F, G, K, and a blue star. Class B, A. This is the true condition of affairs, as may be noticed in Table II., which gives the percentage of the total energy falling upon the thermocouple, which is transmitted by the water-cell, for blue, yellow and red stars. From this table it may be seen that as much as 60 per cent, of all the radiations coming from a blue star lies in the spectral region to which the eye is sensitive, while only from 20 to 30 per cent, of the total radiation received from a red star affects the eye and the photographic plate. This brings out very clearly why it is that a red star of the same visual brightness as a blue star (causes a larger galvanometer deflection) emits from two to three times as much total radiation. It means that from 70 to 80 per cent, of the radiation from a red star lies in the infra-red—beyond the spectral region to which our eyes are sensitive.

The absorption cell tells us nothing of the size or the distance of the star. It indicates that the shape of the spectral energy curve of the star is such that only about one fourth of the, total energy emitted by a red star lies in the visible and in the ultraviolet part of the spectrum. However, it should dispel all doubt as to the quality of the radiations emitted by red and by blue stars. The absorption does more than merely tell us the region of the spectrum in which the most energy is distributed. It may prove useful in detecting dark companions of