utes may be, for instance, the diameter and form of the
particles, their mode of rotation, &c. By these attributes the optical
and electrical properties of the radiation are to be explained. I shall
not here attempt any such explanation, but shall confine myself to the
property which the particles have of possessing a different mode of
deviating from the rectilinear path as they pass from one medium to
another. This deviation depends in some way on one or more attributes of
the particles. Let us suppose that it depends on a single attribute,
which, with a terminology derived from the undulatory theory of
HUYGHENS, may be called the _wave-length_ ([lambda]) of the particle.

The statistical characteristics of the radiation are then in the first
place:--

(1) the total number of particles or the _intensity_ of the radiation;

(2) the _mean wave-length_ ([lambda]_0) of the radiation, also called
(or nearly identical with) the _effective_ wave-length or the colour;

(3) _the dispersion of the wave-length_. This characteristic of the
radiation may be determined from the _spectrum_, which also gives the
variation of the radiation with [lambda], and hence may also determine
the mean wave-length of the radiation.

Moreover we may find from the radiation of a star its apparent place on
the sky.

The intensity, the mean wave-length, and the dispersion of the
wave-length are in a simple manner connected with the _temperature_
(_T_) of the star. According to the radiation laws of STEPHAN and WIEN
we find, indeed (compare L. M. 41[1]) that the intensity is proportional
to the fourth power of _T_, whereas the mean wave-length and the
dispersion of the wave-length are both inversely proportional to _T_. It
follows that with increasing temperature the mean wave-length
diminishes--the colour changing into violet--and simultaneously the
dispersion of the wave-length and also even the total length of the
spectrum are reduced (decrease).

2. _The apparent position of a star_ is generally denoted by its right
ascension ([alpha]) and its declination ([delta]). Taking into account
the apparent distribution of the stars in space, it is, however, more
practical to characterize the position of a star by its galactic
longitude (_l_) and its galactic latitude (_b_). Before defining these
coordinates, which will be generally used in the following pages, it
should be pointed out that we shall also generally give the coordinates
[alpha] and