Kant, 
liambert, 
Wright. 
844 
ably in the Milky Way, and which may in some sense 
be almost said to compose it—are the parent-matter 
of stars, whilst stars abound in their neighbour- 
hood, or seem partly to compose them, in a state 
of incredible aggregation, That the mass of sidereal 
matter composing the Milky Way (which forms an 
irregular girdle round the whole heavens!) has some 
special reference in its locality to our solar system, 
was also an early idea, which, towards the middle 
of the 18th century, had attained to something like 
a definite speculation in the minds of Kant, Lambert, 
and other philosophers, chiefly those of Germany. 
Its origin is perhaps traceable to Thomas Wright of 
Durham? (1742), who affirmed that the Milky Way 
MATHEMATICAL AND PHYSICAL SCIENCE. 
[Diss. VI. 
simplicity of the means by which it was reached, and 
the amount of mere mechanical and arithmetical la- 
bour which forms its sole basis, and constitutes its 
essence, is a powerful lesson of how little mankind 
has a right to scorn the humblest means of knowledge, 
or to attach the prerogative of genius only to the 
lofty flights of imagination and speculative philoso- 
phy.® This triumph belongs to Herschel alone; his 
successors have commented upon and scrutinized his 
results; his son has “ gauged” the Southern Hemi- 
sphere, with like results; but the original result is 
substantially confirmed. We may therefore antici- 
pate so far as to quote the numbers denoting the 
“plenitude” of stars as we recede from the equator 
of the Milky Way in either direction, first, as calcu- as observed 
lated by Struve* and Sir John Herschel, from Sir Wil- by Sir Wil- 
liam Herschel’s observations in the Northern hemi- !i#™ 2nd 
sphere; secondly, as calculated for the Southern he- pbs 
is a projection on the sphere of a stratum of stars, in 
the midst of which our Sun and system are placed— 
somewhat excentrically, however, towards Sirius. 
(202.) I do not know whether Sir W. Herschel derived 
Herschel’s 
“ gauges,” 
(203.) 
Plenitude 
of stars in 
different 
regions of 
the sky, 
his first ideas on the subject either from English or 
German authors. His customary silence (which is 
to be regretted) on the history of his discoveries, 
leaves us in doubt of his originality. Be this as it 
may, he proceeded to test the reasonableness of the 
speculation, in that definite and thorough manner 
which was very characteristic of his genius, He pro- 
ceeded to number the stars after a fashion which, if 
the early attempt of Hipparchus was pronounced in 
its day impious, might well have been accounted, 
even in Herschel’s age, impossible. ‘To use his own 
bold expression, he gauged the heavens, by counting 
the whole number of stars visible in the field of his 
20-feet reflector as many times as possible, and in 
every region of the sky which was visible from 
Slough, distinguishing the results of each region. As 
it was impossible in one lifetime to accomplish the en- 
tire survey (for the complete sphere contains 883,000 
fields of 15’ each in diameter), he took an average for 
each region, and thus determined the general popula- 
tion of the sky in all directions. The result more than 
confirmed the suspicions of Wright and Lambert, 
and demonstrated a remarkable, and, on the whole, 
a steady law of decrease, from the central zone of the 
Milky Way in opposite directions, until we reach two 
poles, one in the Southern, the other in the Northern 
hemisphere, which are the localities poorest in stars. 
This piscovery (for it well deserves the name), 
which assigns a Law to the distribution of the entire 
visible bodies of the Universe in space, is surely one 
of the greatest which man has ever attained. The 
misphere by Sir J. Herschel, from his own observa- 
tions at the Cape of Good Hope. 
Limits of Zone, Average Number of Stars visible in 20-feet 
reckoning from Reflector ; Aperture 18°8 inches ; Field 
Equator of 15’ diameter. 
Milky Way. N. Hemisphere S. Hemisphere 
(Magn. Power 157.) (Magn. Power 180.) 
53-4 59-1 
15 — 30 24-1 26:3 
30 - 45 13°6 135 
45 — 60 82 91 
60 - 75 54 66 
75 — 90 4:3 6-0 
This table shows the extreme condensation of the stars 
round the ‘“‘ Galactic” equator; indeed Struve carries 
the average “ plenitude” of the field as high as 122 
stars in that plane itself, 
According to Herschel’s views, the whole visible fir- 
mament of stars belongs to the nebula of the Milky 
Way, of which our own system forms an infinitesimal 
part. In his paper of 1785, on “ the Constitution of 
the Heavens,” he inferred that the general form and 
comparative limits of this cluster of stars might be 
found from his “ gauges,” by supposing that his tele- 
scope brought into view every star between the specta- 
tor and the surface of the cluster. Supposing the stars 
to be uniformly distributed within the cluster, it is 
easy to see that the number counted in a conical space, 
having an angle of 15’ at its vertex, would give the 
height of the cone, or the depth to which the spectator 
is immersed in the cluster, in that particular direction; 
and thus he endeavours to assign the solid form of the 
cluster and its dimensions, in terms of the distances 
0°- 15° 
1 Its plane is inclined about 63° to the Celestial Equator: its nodes are in 0% 47™ and 12> 47™ of Right Ascension. 
2In his Synopsis of the Universe, a rare work. See Professor De Morgan’s account of Wright, in the Philos. Magazine 
for 1848, xxxii., p. 241. Wright believed that the Milky Way was a congeries of stars of a particular form, which it owes to 
a rotation of the whole round some central point. 
% Another remark, perhaps, deserves to be recorded. Had Hipparchus and his followers been deterred by the threatened 
charge of impiety from making a catalogue of stars, how great the loss would have been, not toscience alone, but to our esti- 
mate of the order and magnificence of creation. The telescope only increased our knowledge of the number of worlds, without 
really enabling us to approach to their limits :—the gauges of Herschel, which he at first supposed to reach to the boundary 
of the visible firmament, proved to him at last that the wealth of creation was unjath 
re he could apply to it. 
ble by any 
“ Numbers numberless” appeared in those regions where the stratum of the Milky Way is most extended. 
4 Etudes d’ Astronomie Stellaire. 
(204.) 
