106 STELLAR NUMBERS AND DISTANCES. 
Particular attention should be paid to the differences of stellar dis- 
tribution upon plates of the Milky Way proper, and of the dark aper- 
ture between its cloven portions. That this really forms an integral 
part of the galaxy is shown by the far greater profusion of small stars 
there than in the general sky at the outer margins of the galactic 
branches—a fact in itself fatal to the “spiral theory,” by which the 
rift was interpreted as a chink of ordinary sky-background left by the 
interlacing, to the eye, of two great streams of stars, one indefinitely 
more remote than the other. From photographs we may now hope to 
learn what is the nature of the distinction between rift and branches— 
what are the magnitudes, relative numbers, and presumable mean dis- 
tances, of the clustering stars present in the latter, but absent from 
former. 
Gauges taken in the neighborhood of the southern “coal-sack” ought 
to prove instructive as to the nature of the nebulous stratum out of 
which it seems as if scooped. If the Milky Way be there shallower 
than elsewhere, a greater uniformity of lustre may be looked for among 
the stars composing it. No background profusely stored with lessen- 
ing ranks will come into view, and stars below the average of those 
grouped in bright masses, representing their genuine companions, will 
be but scantily present. 
Outside the milky way, two points suggest themselves as likely to be 
settled by photographic gauges. Argelander tound that the faintest 
stars in the Durchmusterung were everywhere in excess of their due 
proportion.* Even at the galactic pole, their increase, as compared 
with the class next below, was sextuple instead of quadruple; in the 
undivided galactic stream it was nine and a half, in the rift eight and 
a half times. If this semblance of crowding in all directions at about 
the mean distance of a ninth magnitude star be no accident of enumera- 
tion, then the milky way is only the enhancement of a phenomenon 
universally present, and the fundamental plan of the sidereal system 
must be regarded as that of a sphere with superficial condensation in- 
tensified in an equatorial ring. The counts, to settle this question, will 
have to extend over a considerable area. 
The second point for photographie investigation refers to the limits 
of the system towards the galactic poles. There is reason to believe 
them comparatively restricted. M. Celoria, of the Milan observatory, 
using a refractor capable at the utmost of showing stars of eleventh mag- 
nitude, obtained for a “‘mean sounding,” at the north pole of the milky 
way, almost identically the same number given by Herschel’s great re- 
flector.t That is to say, no additional stars were revealed by the larger 
instrument. Should this evidence be confirmed, the boundary of the 
stellar scheme should here be placed at a maximum remoteness of 3,500 
years of light travel. 
* Bonner Beobachtungen, Ba. v, ‘ Einleitung.” 
t Memorie del” Instituto Lombardo, t. X1v, p. 86. 
