12. We have found in §9 that the light-radiation of a star is described by means of the total intensity (_I_), the mean wave-length (λ_0) and the dispersion of the wave-length (σ_λ). λ_0 and σ_λ may be deduced from the spectral observations. It must here be observed that the observations give, not the intensities at different wave-lengths but, the values of these intensities as they are apprehended by the instruments employed--the eye or the photographic plate. For the derivation of the true curve of intensity we must know the distributive function of the instrument (L. M. 67). As to the eye, we have reason to believe, from the bolometric observations of LANGLEY (1888), that the mean wave-length of the visual curve of intensity nearly coincides with that of the true intensity-curve, a conclusion easily understood from DARWIN's principles of evolution, which demand that the human eye in the course of time shall be developed in such a way that the mean wave-length of the visual intensity curve does coincide with that of the true curve (λ = 530 μμ), when the greatest visual energy is obtained (L. M. 67). As to the dispersion, this is always greater in the true intensity-curve than in the visual curve, for which, according to §10, it amounts to approximately 60 μμ. We found indeed that the visual intensity curve is extended, approximately, from 400 μμ to 760 μμ, a sixth part of which interval, approximately, corresponds to the dispersion σ of the visual curve. In the case of the photographic intensity-curve the circumstances are different. The mean wave-length of the photographic curve is, approximately, 450 μμ, with a dispersion of 16 μμ, which is considerably smaller than in the visual curve. 13. Both the visual and the photographic curves of intensity differ according to the temperature of the radiating body and are therefore different for stars of different spectral types. Here the mean wave-length follows the formula of WIEN, which says that this wave-length varies inversely as the temperature. The total intensity, according to the law of STEPHAN, varies directly as the fourth power of the temperature. Even the dispersion is dependent on the variation of the temperature--directly as the mean wave-length, inversely as the temperature of the star (L. M. 41)--so that the mean wave-length, as well as the dispersion of the wave-length, is smaller for the hot stars O and B than for the cooler ones (K and M types). It is in this manner possible to determine the temperature of a star from a determination of its mean wave-length (λ_0) or from the dispersion in λ. Such determinations (from λ_0) have been made by SCHEINER and WILSING in Potsdam, by ROSENBERG and others, though these researches still have to be developed to a greater degree of accuracy.