SCIENCE, 



[Vol. XVIII. No. 445 



a Centauri, or the smallest stars visible in it with a tele- 

 scope as large as the Washington 26inch refractor must be 

 larger than our sun, a supposition at which the mind rebels 

 when weremember the vast size this would imply for the larger 

 stars evidently involved iu or associated with the Milky 

 Way. For example, in the Pleiades group there are ob- 

 servable with the eye at the telescope a range of some thir- 

 teen magnitudes of the photometric scale, which, translated 

 into ordinary language, means that the larger stars of the 

 cluster give more than a hundred and fifty thousand times 

 as much light as the smaller stars of the cluster. 



In the photographs of the Pleiades cluster we have evi- 

 dence of a range of at least fifteen magnitudes, which means 

 that the larger stars give a million times as much light as 

 the smaller stars, and in the photograph of the coal sack re- 

 gion of the Milky Way there is evidence of a still greater 

 range of magnitudes. The star a Crucis, which is of 1.3 

 magnitude, is evidently associated with a dense cluster of 

 small stars, branches from which can be traced far across 

 the coal-sack region, and extending to a considerable distance 

 over the Milky Way or into the Milky Way to the north of 

 a Crucis. We seem to have in this instance evidence of a 

 range of at least seventeen magnitudes. « Crucis is a dou- 

 ble star with components about five seconds apart, and there 

 are several small companions that have been observed in 

 the telescope. In the glass photograph by Mr. Eussell the 

 spurious disk of the large star is, when examined with a 

 magnifier, seen to contain several small stars forming a clus- 

 ter about the large one. Indeed, some seven or eight of 

 these small stars may be recognized with a magnifying-glass 

 on the edge of the spurious disc of the large star. 



Though the mind may at first be staggered by the concep- 

 tion of stars giving a million times as much light as our sun, 

 we are not in a position to deny the existence of such vast 

 sun-like bodies Indeed those who accept the nebular hy- 

 pothesis as giving the most probable explanation of the ori- 

 gin, or rather of the birth, of the planets of the solar sys- 

 tem, must be prepared to believe that there was a time when 

 the sun had a diameter as large, or nearly as large, as the 

 ' diameter of the orbit of Neptune. If before these more than 

 geologic ages of radiation into space the surface or photo- 

 sphere of the solar mass did not shine as brightly as it shines 

 now, it must, at least, have been a nebula with a very defi- 

 nite surface, which, as seen from a distance a hundred times 

 as great as that of a Centauri would have presented a disc 

 nearly half a second in diameter. No disc has at present 

 been observed to any star; we may therefore feel some con- 

 fidence that there is no such vast sun-like body within a 

 distance from us equal to fifty times the distance of a Cen- 

 tauri. 



In the forthcoming part of the ''Old and New Astron- 

 omy," I have shown reason to believe that there is evidence 

 of absorption of light in space, and that we can, from the 

 numbers of the stars of the various magnitudes, make a 

 rough minimum estimate as to the amount of absorption of 

 light in space, due either to a want of perfect elasticity in 

 the light-transmitting ether, or to dark bodies cutting out or 

 obliterating the light in its passage through space. This 

 greatly reduces our idea of the magnitude of the region we 

 can explore with the telescope and with the camera, — a 

 Centauri would probably be lost to the Lick telescope if it 

 were removed to three hundred times its present distance, — 

 and it also greatly reduces our idea of the distance of the 

 small stars of the Milky Way, and of the scale of the galac- 

 tic system as well as of the nebular system and of the system 



of clusters, red stars, and bright line stars which asre so evi- 

 dently associated with it. 



It is not so very long ago that it was generally taught that 

 the nebulffi were galaxies of stars more or less similar to the 

 Milky Way that surrounds us, but so inconceivably remote 

 as to appear when observed with the largest telescopes like 

 small spots in the heavens. This theory suited the popular 

 taste, and died hard. It involved the assumption that man 

 could explore with the instruments at his disposal a space 

 so immense that the interstellar spaces which we can just 

 measure or guess at, are dwarfed into points beside the dis- 

 tance from which light travels to us. 



The theory should have been disposed of by the observa- 

 tions of Sir William Herschel, who noted that many nebulae 

 are evidently associated with stars, and observed that the 

 smaller nebulae wei'e distributed over the heavens in a man- 

 ner which shows an intimate connection between them and 

 the brighter stars. He noted that the nebulse in the north- 

 ern heavens were clustered in the pole of the Milky Way, 

 and descended like a canopy on all sides, leaving a dark 

 space or channel separating the nebulous region from the 

 rich stellar region of the Milky Way. Sir William Her- 

 schel also fully satisfied himself that "there were nebulosi- 

 ties which are not of a starry nature," and from his observa- 

 tions of diffused nebulae he formed his well-known hypothe- 

 sis of a diffused luminous fluid which, by its eventual 

 aggregation, produced stars. But he did not proceed to the 

 legitimate deduction from his observations as to the general 

 distribution of nebulse, viz., that nebulae which are arranged 

 so symmetrically with respect to the stars must belong to 

 the stellar system, and therefore cannot be assumed to lie at 

 immense distances compared with the distance of the Milky 

 Way stars. 



Sir John Herschel extended the observations of his father 

 to the southern heavens, and showed that there was a simi- 

 lar clustering of the smaller nebulae on the southern side of 

 the Milky Way, and a similar intimate connection between 

 the distribution of stars and the distribution of nebulae in 

 the southern hemisphere (Cape Observations, p. 134) ; but it 

 was not until 1858 that the obvious conclusion from these 

 observations was drawn by Mr. Herbert Spencer in a remark- 

 able paper on " The Nebular Hypothesis," published in the 

 Westminster Review. He remarked, ' ' If there were but 

 one nebula, it would be a curious coincidence were this one 

 nebula so placed in the distant regions of space as to agree 

 in direction with a starless spot in our own siderial system.* 

 If there were but two nebulae, and both were so placed, the 

 coincidence would be excessively strange; what then shall 

 we say on finding that there are thousands of nebulae so 

 placed? Shall we believe that in thousands of cases these 

 far-removed galaxies happen to agree in their visible posi 

 tions with the thin places in our own galaxy? Suchahelief 

 is impossible." 



Mr. Herbert Spencer's paper was not illustrated by charts, 

 and the force of his reasoning was not generally perceived 

 till some ten years afterwards, when Professor Cleveland 

 Abbe drew attention in the Monthly Notices of the Royal 

 Astronomical Society for May, 1867, to the intimate connec- 

 tion between the distribution of nebulae in space and stars; 

 and Mr. Proctor, in 1869, constructed some charts on an 

 equi-surface projection, which graphically put his readers in 

 possession of the facts and carried conviction to all who read 

 his remarks. 



The theory that the nebulae were distinct galaxies involved 

 the assumption that light can reach us from regions many 



