August 1, 1891.] 



KNOWLEDGE. 



155 



of 39-81 inches aperture. That is, it would not be visible 

 in the great Lick 36-inch refractor. 



These calculations are based on the assumption that 

 there is no absorption of light in passing through great 

 distances of space, and also on the assumption that there 

 is no loss of hght in passing through such thick lenses. 

 The thickness of the object-glass of the " Washington " 2G- 

 inch refi-actor at its centre is nearly three inches ; thus, 

 the flint glass lens is there 0-96 inch thick, while the 

 crown glass lens is 1-88 inch thick at its centre. Buch a 

 thickness more than halves the intensity of the emergent 

 pencil ; and the loss of light by absorption in passing 

 through the glass near the centre of the Lick object-glass 

 must be considerable. Exact measures of the absorption 

 of light by such great lenses would be of much interest. 

 We may, however, probably assume with some con- 

 fidence, that if a." Centmiri were removed to twelve 

 hundi-ed times its present distance it would not be 

 visible in the Lick telescope, even though there were 

 no absorption of light in space ; and a? Centauri is 

 probably larger and brighter than our Sun.''' 



Stars smaller than our Sun would he lost to sight at 

 smaller distances. Consequently the Milky Way must 

 either be nearer to us than a thousand times the distance 

 of a. Centauri, or the smallest stars visible in it with a 

 telescope as large as the Washington 2G-incli refractor 

 must be larger than our Sun — a supposition at which 

 the mind rebels when we remember the vast size which 

 this would imxjly for the larger stars evidently involved in 

 or associated with the Milky Way. For example, in the 

 Pleiades group (which we showed reason in the May 

 number for believing to be associated with the Milky 

 Way) there are observable with the eye at the telescope 

 a range of some thirteen 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 

 evidence 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 region of the Milky Way, published in the .June 

 number, there is evidence of a still greater range of magni- 

 tudes. The star a. Crucix, which is of the 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 CnuiK. We seem to ha%e in this instance evidence of a 

 range of at least seventeen magnitudes. And the proof of 

 the connection between the large star and the small stars 

 of the cluster is far stronger than as stated by me in the 

 May number, a ' Vi((/,sisa double star with components about 

 5 seconds apart and there are several small companions that 

 have been observed in the telescope, in the glass photo- 

 graph sent mo by Mr. llussell the spurious disc of the large 

 star is, when examined with a magnifier, seen to contain 

 several small stars forming a cluster about the large one. 

 Lideed, in the plate published in the May number 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 

 conception of stars giving a million times as much light 



* Assiiiiiing witli Mr. Govo a i)priocl of 77 yi'iii's for tins biiuii\v, 

 and a |>iu'allax of '7.3 of a uccond, tlie sum of llie musses of tlic 

 compoiicuts will be 2'1'1 times tbo mass of tlic Sun. 



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 hypothesis as giving the most probable 

 explanation of the origin, or rather of the birth of the 

 planets of the Solar system, 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 photosphere of the 

 solar mass did not shine as brightly as it shines now, it 

 must, at least, have been a nebula with a very definite 

 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 

 confidence that there is no such vast sun-like body within 

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

 Centauri. 



In the forthcoming part of the Old and New Astrojwmy, 

 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 ^^'ay, and of the 

 scale of the galactic system as well as of the nebular 

 system and of the system of clusters, red stars, and 

 bright line stars which are so evidently associated 

 with it. 



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

 that the nebula) 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 distance from 

 which light travels to us. 



The theory should have been disposed of by the obser- 

 vations of Sir William Herschel, who noted that many 

 nebuliE are evidently associated with stars, and observed 

 that the smaller nebulae were distributed over the heavens 

 in a manner which shows an intimate conncctidn between 

 them and the brighter stars. He noted that the nebuh* 

 in the northern 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 WLUiam Herschel also fully satisfied himself that 

 " there were nebulosities which are not of a starry 

 nature," and from his observations of diffused nebulic 

 he formed his well-known hypothesis of a dift'used 

 luminous fluid which, by its eventual aggregation, pro- 

 duced stars. But he did not proceed to tlie legitimate 

 deduction from his observations as to the general distribu- 

 tion of nebuhe, viz. : that nebula' 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. 



