10 SEE— DETERMINATION OF THE [Januarys. 



of the great Sir William Herschel should have been neglected all 

 this time! Will it seem credible to future ages that such a remark- 

 able retrogradation of opinion could have occurred and persisted 

 during the nineteenth and twentieth centuries? If so, it must be 

 attributed to the narrowing effects of extreme specialization, which, 

 with the advance of science, has been difificult to avoid in our time, 

 and yet is utterly disastrous to the growth of true natural philosophy 

 as the study of nature in the widest sense. 



§6. Other Methods for Confirming the Great Depth 



OF the Milky Way. 



(a) The girdle of helium stars about our sun, according to the 

 Lick determination, has a mean distance of 540 light-years, or a 

 mean diameter of 1,080 light-years. If this be one twentieth of the 

 average thickness of the Milky Way stratum, as one may infer from 

 the appearance of certain clusters in the constellation Sagittarius, 

 which are near enough to be studied intelligently, then we have 

 21,600 light-years for the average thickness of the ]\Iilky Way. 



Now when we traverse the Milky Way from Centaurus to Cepheus, 

 over an arc of 180° in length, the central band appears to the naked 

 eye to have a width of 3° or 4°, as was long ago remarked also by 

 Herschel and Struve. This is an extension along the circle of the 

 Galaxy of about 60 times its thickness. If then the thickness be 

 21,600 light-years, the double depth of the stratum in both directions 

 becomes about two thirds of 21,600 X 60=864,000 light-years. And 

 if only the faint or distant telescopic stars be considered, the width 

 of their belt of distribution is narrower, and the depth would be 

 found several times greater yet. Wherefore it seems certain that 

 the profundity of the Milky Way, considerably exceeds a million 

 light-years, and may be several times that depth. 



(b) Accordingly if we make the very moderate hypothesis that 

 the width of 3° or 4°, which was also noticed by Herschel and 

 Struve, represents chiefly the nearer portion of the Galaxy; and 

 that the remoter portion has a width not exceeding 1°, we should 

 conclude that the depth may be found by multiplying the thickness 



