The numbers on the planisphere given in each area thus express the star density of the region, or the number of stars per 100 square degrees, expressed generally to the nearest unit, the half unit being sometimes added.
A study of the reproduction which we give will show how fairly well the Milky Way may be traced out round the sky by the tendency of those stars visible to the naked eye to agglomerate near its course. In other words, were the cloud forms which make up the Milky Way invisible to us, we should still be able to mark out its course by the condensation of the stars. As a matter of interest, I have traced out the central line of the shaded portions of the planispheres as if they were the galaxy itself. The nearest great circle to the course of this line was then found to have its pole in the following position:
R.A.; 12h. 18m.
Dec. 27°.
This estimate was made without having at the time any recollection of the position of the galaxy given by other authorities. Compared with the positions given in the last chapter by Gould and Seeliger, it will be seen that the deviation is only 5° in right ascension, while the declinations are almost exactly similar. We infer that the circle of condensation found in this way makes an angle with the galaxy of less than 5°.
DISTRIBUTION OF THE FAINTER STAES.
The most thorough study of the distribution of the great mass of stars relative to the galactic plane has been made by Seeliger in a series of papers presented to the Munich Academy from 1884 to 1898. The data on which they are based are the following:
1. The Bonner Durchmusterung of Argelander and Schonfeld, described in our third chapter. These two works included under this title are supposed to include all the stars to the ninth magnitude, from the north pole to 24° of south declination. But there are some inconsistencies in the limit of magnitude which we shall hereafter mention.
2. The 'star gauges' of the two Herschels. These consisted simply in counts of the number of stars visible in the field of view of the telescope when the latter was directed toward various regions of the sky. Sir William Herschel's gauges were partly published in the 'Philosophical Transactions.' A number of unpublished ones were found among his papers by Holden and printed in the publications of the Washburn Observatory, Vol. II. The younger Herschel, during his expedition to the Cape of Good Hope, continued the work in those southern regions of the sky which could not be seen in England.
3. A count of the stars by Celoria, of Milan, in a zone from the equator to 6° Dec, extended round the heavens.
