spectrum, this displacement being towards the red or the violet
according as the star is receding from or approaching the observer.

The velocity _W_ will be expressed in siriometers per stellar year
(sir./st.) and alternately also in km./sec. The rate of conversion of
these units is given in Sec.5.

17. Summing up the remarks here given on the apparent attributes of the
stars we find them referred to the following principal groups:--

I. _The position of the stars_ is here generally given in galactic
longitude (_l_) and latitude (_b_). Moreover their equatorial
coordinates ([alpha] and [delta]) are given in an abridged notation
([alpha][delta]), where the first four numbers give the right ascension
in hours and minutes and the last two numbers give the declination in
degrees, the latter being printed in italics if the declination is
negative.

Eventually the position is given in galactic squares, as defined in Sec.2.

II. _The apparent motion of the stars_ will be given in radial
components (_W_) expressed in sir./st. and their motion perpendicular to
the line of sight. These components will be expressed in one component
(_u_0_) parallel to the galactic plane, and one component (_v_0_)
perpendicular to it. If the distance (_r_) is known we are able to
convert these components into components of the linear velocity
perpendicular to the line of sight (_U_ and _V_).

III. _The intensity of the light_ of the stars is expressed in
magnitudes. We may distinguish between the _apparent_ magnitude (_m_)
and the _absolute_ magnitude (_M_), the latter being equal to the value
of the apparent magnitude supposing the star to be situated at a
distance of one siriometer.

The apparent magnitude may be either the _photographic_ magnitude
(_m'_), obtained from a photographic plate, or the _visual_ magnitude
(_m_) obtained with the eye.

The difference between these magnitudes is called the _colour-index_
(_c_ = _m'_-_m_).

IV. _The characteristics of the stellar radiation_ are the mean
wave-length ([lambda]_0) and the dispersion ([sigma]) in the
wave-length. _The mean wave-length_ may be either directly determined
(perhaps as _effective_ wave-length) or found from the spectral type
(spectral index) or from the colour-index.

There are in all eight attributes of the stars which may be found from
the observations:--the spherical position of the star (_l_, _b_), its
distance (_r_), proper motion (_u_0_ and _v_0_), radial v