tood 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 ([lambda] = 530 [mu][mu]), 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 Sec.10, it amounts to approximately 60
[mu][mu]. We found indeed that the visual intensity curve is extended,
approximately, from 400 [mu][mu] to 760 [mu][mu], a sixth part of which
interval, approximately, corresponds to the dispersion [sigma] 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 [mu][mu], with a dispersion of 16 [mu][mu], 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 ([lambda]_0) or from the dispersion in [lambda].
Such determinations (from [lambda]_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.

14. _Effective wave-length._ The mean wave-length of a spectrum, or, as
it is often called by the astronomers, the _effective_ wave-length, is
generally determined in the following way. On account of the refraction
in the air the image of a star is, without the use of a spectroscope,
really a spectrum. After some time of exposure we get a somewhat round
image, the position of which is determined precisely