618 
centre ; that there were several heavens round it: the 
heaven of the moon, the heaven of Venus, of Mars, and 
so on, till at last there was a heaven of the stars, a crystal- 
line sphere to which the stars were fixed like golden nails. 
Let the glass globe represent this crystalline sphere. 
The Milky Way is a great circle inclined, at an angle 
of about 62°, to the earth’s equator or to the equatorial 
plane extending to the stars. We know nothing, of 
course, of the reason for that angle of 62°, but it has its 
importance, because not only must the belt cross the 
equator at two opposite points, as it does in two opposite 
constellations, Aquila and Monoceros, but the poles of 
the Milky Way must lie at the points of greatest distance 
from the junction with the equator, in certain constel- 
lations. These are Coma Berenices and Sculptor, and the 
position of the N. galactic pole, as the pole of the Milky 
Way is called, is in R.A. 12h. 4om. Dec. + 28°. Now, 
although the Milky Way looks very unlike the other parts 
of the heavens, we have known since the time of Galileo 
that the difference arises from the fact that it is composed 
of a tremendous multitude of stars ; and this is why I have 
drawn attention to it, a very large percentage of the 
masses of matter which compose our system lies in the 
plane of the Milky Way. It does not merely represent a 
fiery or igneous fluid, as different schools thought it did in 
the old days. So far as our opera-glasses and telescopes 
indicate to us, we are in presence of an innumerable 
multitude of stars. When, however, we come to look at it 
a little more closely, we find that from two points in it 
branches are thrown out, so that over some part of its 
orbit, so to speak, it is double ; there isa distinct doubling 
of the Milky Way along a partof its length. But in spite 
of that, the middle line of the galaxy or the Milky Way 
is really not distinguishable from a great circle, as was 
formerly supposed. The great rift which separates these 
two parts of it begins near a star in the southern hemi- 
sphere, a Centauri, and it continues for more than six 
hours in right ascension until the two branches meet 
again in the constellation Cygnus, which is well within our 
ken in the northern heavens. The distance apart of the 
middle lines of these two components of the Milky Way 
where the split is most obvious is something like 17°, so 
that, in addition to the angle of 62° from the ecliptic, in 
some part of the Milky Way, there is another offshoot 
springing out of it at an angle of something like 17. 
The regions of greater brilliancy correspond approxi- 
mately to the places where the branches intersect each 
other. In short, there are sundry indications that the 
whole phenomena of the Milky Way may become 
simplified by treating it as the resultant of two super- 
imposed galaxies. The general view till recently was 
that the Milky Way is not a great circle, because it was 
thought the sun was not situated in its plane. The whole 
mass of stars was likened to a millstone split along one 
edge, which was Sir William Herschel’s first idea. But 
the recent work, chiefly of Gould in Argentina, has shown 
that it practically is a great circle. However that may 
be, in one part of the heavens this wonderful Milky Way 
appears as a single, very irregular stream, and in another 
part it appears to be duplicated. 
It is impossible in this short course of lectures to 
attempt to give anything like an historical statement 
of the growth of our knowledge of the Milky Way. 
I can only refer you to the Milky Way itself ; and the 
next time any of you have an opportunity of seeing it, 
just look at the wonderful majesty and complexity of it. 
We find in it indications of delicate markings going 
out into space, apparently coming back strengthened, of 
streams in all directions, of clusters clinging to those 
streams, and so on. In other parts it is curdled, which 
is the only term which I can use to express my meaning. 
In another part we may find it absolutely free from any 
important stars; in another we may find it mixed with 
obvious nebula ; and in another we may find it mixed, not 
NO. 1565, VOL. 60] 
NATURE 
[OcToBER 26, 1899 
only with obvious nebula, but with a great number of 
bright-line stars involved, not only in the Milky Way, but 
in the nebula itself. 
We have now, fortunately for science, priceless photo- 
graphs of these different regions. One will give us an 
idea of the enormous number of stars in some parts ; 
another one of the streams of nebrlous matter which are 
seen in the Milky Way from region to region. Again we 
find a regular river of nebulous matter rushing among 
thousands of stars. In some the galaxy seems to tie itself 
in knots. There is an individuality in almost every part 
of it, which we can study on our photographic plates ; 
practically there are no two parts alike. Others again 
bring before us the curdled appearance which is visible 
in different regions, and finally the connection of the 
infinite number of stars with obvious nebulous matter. In 
this way, then, we are enabled to form an idea of the 
general conditioning of things as we approach the 
Milky Way. 
The next important point is that the enormous in- 
crease of stars in the Milky Way is not limited to the 
plane itself, but that there is really a gradual increase 
from the poles of the Milky Way, where we get the 
smallest number of stars. It is not very easy to bring 
together all the information, for the reason that different 
observers give different measures; they take different 
units for the space they have determined to be occupied 
by stars from the pole towards the galactic plane ; and 
also the number of stars in the northern hemisphere is 
not the same as the number in the southern hemisphere. 
But roughly speaking we may say, if we represent the 
number of stars at the galactic pole by four, the number 
of stars in the galactic plane will be about fifty-four. 
The following table will show the gradual increase in 
the number of stars from the pole to the plane, as seen 
by the Herschels with a reflecting telescope of eighteen 
inches aperture and twenty feet focal length :— ! 
Average number of stars per field of 15’ 
Galactic polar distance. 3 are 
Northern. Southern. 
° wy | 
O-15 4°32 | 605 
WS 32) 5°42 6°62 
30-45 8°21 9 08 
45-60 13°61 | 13°49 
60-75 24°09 | 26°29 
75-90 53 43 59°06 
A consideration of the distribution of stars in Right 
Ascension between declinations 15° N. and 15° S. led 
Struve to the conclusion that there are well marked 
maxima in R.A. 6h. 4om, and 18h. 4om., and minima in 
R.A. th. 30m. and 13h. 3om.; he remarks that the 
maxima fall exactly on the position of the Milky Way in 
the equator, and further states that “the appearance of 
the close assemblage of stars or condensation, 1s closely 
connected with the nature of the Milky Way, or that this 
condensation, and the appearance of the Milky Way, are 
identical phenomena.” 
* Although the Milky Way dominates the distribution of 
stars, and especially of the fainter stars, it does not appear 
to be the only ring of stars with which we have to do. 
Sir John Herschel traced a zone of bright stars in the 
southern hemisphere, which he thought to be the projec- 
tion of a subordinate shoot or stratum. That was the 
first glimpse of a new discovery, which was subsequently 
established by Dr. Gould in his work in the southern 
hemisphere at Cordova. He found that there was a 
stream of bright stars to be traced through the entire 
circuit of the heavens, forming a great circle as well de- 
1 Outlines of Astronomy, Herschel, pp. 535, 536- 
