46 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



Kant, 

 Lambert, 



Wright. 



(202.) 

 Herschel's 



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 1 ) 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 2 (1742), who affirmed that the Milky Way 

 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. 



I do not know whether Sir W. Herschel derived 

 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 DISCOVERY (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 



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. 3 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- a s observed 

 lated by Struve* and Sir John Herschel, from Sir Wil- by Sir \\ H- 

 liam Herschel's observations in the Northern hemi- ^ iam a " u 

 sphere ; secondly, as calculated for the Southern he- Herschel. 

 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.) fUagn. Power 180.) 



0- 15 53-4 59-1 



15 - 30 24-1 26-3 



30 - 45 13-6 13-5 



45 - 60 8-2 9-1 



60 - 75 6-4 6-6 



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- (204.) 

 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 



1 Its plane is inclined about 63 to the Celestial Equator : its nodes are in O h 47 m and 12 h 47 m of Right Ascension. 



2 In 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. 



3 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 to science 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 unfathomable by any measure he could apply to it. 

 " Numbers numberless" appeared in those regions where the stratum of the Milky Way is most extended. 



* Etudes d'Astronomie Stellaire. 



