+-------------------+----------------+ | Spectral | Colour-index | | type | index | | +-------+-----------+----------------+ | B0 | -1.0 | -0.46 | | B5 | -0.5 | -0.23 | | A0 | 0.0 | 0.00 | | A5 | +0.5 | +0.23 | | F0 | +1.0 | +0.46 | | F5 | +1.5 | +0.69 | | G0 | +2.0 | +0.92 | | G5 | +2.5 | +1.15 | | K0 | +3.0 | +1.38 | | K5 | +3.5 | +1.61 | | M0 | +4.0 | +1.84 | +-------+-----------+----------------+ TABLE 1*. _GIVING THE MEAN SPECTRAL INDEX CORRESPONDING TO A GIVEN COLOUR-INDEX._ +----------------+-------------------+ | Colour-index | Spectral | | | index | type | +----------------+---------+---------+ | | | | | -0.4 | -0.70 | B3 | | -0.2 | -0.80 | B7 | | 0.0 | +0.10 | A1 | | +0.2 | +0.50 | A5 | | +0.4 | +0.90 | A9 | | +0.6 | +1.30 | F3 | | +0.8 | +1.70 | F7 | | +1.0 | +2.10 | G1 | | +1.2 | +2.50 | G5 | | +1.4 | +2.90 | G9 | | +1.6 | +3.30 | K3 | | +1.8 | +3.70 | K7 | | +2.0 | +4.10 | M1 | +----------------+---------+---------+ From each catalogue of visual magnitudes of the stars we may obtain their photographic magnitude through adding the colour-index. This may be considered as known (taking into account the high coefficient of correlation between _s_ and _c_) as soon as we know the spectral type of the star. We may conclude directly that the number of stars having a photographic magnitude brighter than 6.0 is considerably smaller than the number of stars visually brighter than this magnitude. There are, indeed, 4701 stars for which _m_ < 6.0 and 2874 stars having _m′_ < 6.0. 16. _Radial velocity of the stars._ From the values of α and δ at different times we obtain the components of the proper motions of the stars perpendicular to the line of sight. The third component (_W_), in the radial direction, is found by the DOPPLER principle, through measuring the displacement of the lines in the 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 §5. 17. Summing up the remarks here given on the apparent attributes of the stars we find them referred to the following principal groups:--