October 1, 1892.] 



KNOWLEDGE 



191 



one or the other of the stars, or in our solar system." 

 But iu 178 1 Miohell pointed out that the great and 

 iaereasing number of known double stars could leave no 

 doubt in any properly-balanced mathematical mhid (even 

 if his former paper, in spite of its lesser basis of knowledge, 

 were not sufficiently convincing j " that by far the greater 

 part, if not all of them, are systems of scars so near each 

 other as probably to be liable to be affected sensibly by 

 their mutual gravitation ; and it is therefore not unlikely 

 that the periods of the revolutions of some of these about 

 their prmcipals (the smaller ones being on this hypothesis 

 considered as satellites of the others) may some time or 

 other be discovered." * Very little notice seems to have 

 been taken of this confident prediction. Twenty years 

 ])assed, and the scientific world exclaimed. How wondciful .' 

 when Herschel's observations forced him to acknowledge 

 that many of these double stars were really binary 

 systems, whose components revolve about each other in 

 tlie same way as the earth and moon. 



All tliis is in Michell's one paper of 17671 . Had he 

 never after put pen to paper, nor engaged in any further 

 ssientihc pursuits, enough had been accomplished to place 

 him in the front rank of English philosophers. Not only 

 did he invent his own method of procedure in his researches, 

 but as its result he confidently announced the solution of 

 problems which men of that day almost thought it pre- 

 sumptuous to speak of. The theory of probabilities not 

 being then appreciated and relied on as now, there seemed 

 more of shadow than of substance iu his speculations. 

 More attention would have been accorded him by his con- 

 temporaries, perhaps, had he, like Hersohel, done some- 

 thing sensational. Herschel attracted little attention until 

 his discovery of Uranus burst upon the world. But 

 Michell did nothing to take the world by storm. 

 (To be iOHti)nu-d.) 



* Phil. Trans. 



+ Mr. Sutton has not referred to what seems to me to be the most 

 striking and remarkable suggestion in this pajiev, in whieh Miebell 

 antieipates Piekering's and Monek's methods of comparizig tiie 

 brightness or density of double stars. It is contained in a note on 

 page 238 of his paper, and most concisely points out how, if we know 

 the period and brightness of a double star, we may neglect its parallax, 

 and directly compare its brightness anil mass with that of the sun. 

 Prof. E. C. Pickering, in 1880, sliowed how, iu dealing with a binary 

 star system, the parallax might be neglected, and the density or bright- 

 ness of the double st.ir might be comparod directly with th;it of the 

 sun. Mr. Monck. in the Ob.ie/-rafoi\i/ of 1887, showed how the 

 brightness or ^lensity of one binary system might be compared with 

 that of another binary system. Neither Pickering nor Monck seeiu 

 to have known of MicheU's note which anticipates their whole line 

 of reasoning. It runs as follows : — 



" If, however, it shoulrl hereafter be found that auy of the stars have others 

 revolving about Ibem (for uo satellites shinini? by a borrowed liijht could 

 pjssibly by visible), we should then have the means of discovering the 

 proportion between the li^ht of the sun and the light of those stars, relatively 

 to their respective quantities of matter, for in this case tlie times of the 

 revelations and the greatest apparent elongations of those stars that revolved 

 about the others as satellites being known, the relation between the appai-ent 

 diameters and the densities of the central stars would be given, whatever was 

 their distance from us, and the actual quantity of matter wliich they contained 

 wtiuid be known whenever their distance was known, being greater or less in 

 pr.>i>i)rtion to the cube of that distance. Hence, supposing them to be of the 

 same deusit,v with the sun, the proportion of the brightness of their siu-faces, 

 compared with that of the sun, would be known from the comparison of the 

 wlioW of the light which we receive from them with that wliich we receive 

 from the sun ; but if they should happen to be either of greiter or less density 

 than the sun, the whole of their light not being atfected by these suppositious, 

 their surfaces woald, indeed, be more or less Inmiuous, acordingly as they 

 were upou this account less or greater, but the quantity of light corresponding 

 to the same quantity of matter would still remain the same. 



" The apparent distances at which satellites would revolve about auy stars 

 would be equal to the semi-annual parallaxes of those stars, seen from planets, 

 revolving about the sun in the same periodical times with themselves, supposing 

 the parallaxes to be such as they would be if the stars were of the same size 

 and density with the sun." 



At the date of Michell's paper (1767) less than seventy close double 

 stars were known, and none were known to be moving about one 

 another, but in this paper, as well as in a second paper published in 

 the Phil. TraiLi. for 1781, he speaks with confidence as to the 

 pliysical connection between such closely situated pairs. — A. C. 

 Ranyaed. 



WHAT IS A NEBULA? 



By A. C. Banyard. 



THERE can be little doubt that the nebulse are, as a 

 general rule, very transparent, for it cannot 

 reasonably be doubted that by far the greater 

 number of the stars which appear to us surrounded 

 by nebulous matter are really involved in the 

 brighter and central parts of the nebuliB which appear to 

 surround them, and are not merely seen by chance 

 projected on a nebulous background. A very elementary 

 application of the doctrine of chances will show the 

 enormous improbability of the hundreds of nebulous stars 

 known, all being seen projected (generally pretty centrally) 

 upon a nebulous background. In the ease of the great 

 nebula in Orion, and of the Pleiades nebula, as well as in 

 some of the smaller nebulfe, we actually see nebulous 

 structures which appear to spring from stars or groups of 

 stars like the trapezium in the Orion nebula ; the nebulous 

 structures grow gradually fainter as the distance from the 

 star from which they appear to spring increases, and in some 

 instances the nebulous structures branch or divide in a 

 direction away from the star in a manner which leaves no 

 room for doubting that the seat of origin of the structure 

 (that is, the place from which it was belched forth) 

 must have been within the star. 



We may therefore feel practically certain that we receive 

 the light from many stars after its passage through many 

 thousands of millions of miles of nebulous matter. We 

 know how the light of our own sun is dimmed at sunset 

 and sunrise by its passage through a few hundred miles of 

 our atmosphere, so that the eye can easily gaze on the 

 sun's disc ; and a photograph which at midday can be 

 obtained in a fraction of a second, takes at sunset or sim- 

 rise many seconds, or even some minutes, to give it a 

 suitable exposure. The almost perfect transparency of 

 this nebuloits matter will be best realized by the student 

 of physics who knows that if half the light were absorbed 

 in its passage through ten million miles of nebula, only a 

 quarter of the light radiated by the star would get through 

 twenty million miles of similar nebulous matter, and only 

 an eighth part through thirty million miles, only a six- 

 teenth through forty million miles, and so on ; for each 

 succeeding ten million miles of similar nebulous haze 

 would halve the light which had succeeded in getting 

 through the nebulous veil between it and the source of light. 

 Thus at a distance of '200 million miles from the nebulous 

 star the star's light would be reduced to about one unllionth 

 of the emitted light; at a distance of 400 million miles it 

 would be reduced to a millionth of a millionth of its original 

 brightness ; and after passing through 1000 million miles 

 its light would have been reduced 75 magnitudes in the 

 stellar scale of brightness. 



Such vast distances are triflingly small when measured 

 by the scale of architecture on which nebultB are built. A 

 nebula a thousand million miles in diameter at the distance 

 of a Centauri (our nearest neighbour amongst tlie stars) 

 would only appear to have a diameter of 8'12" seconds of 

 arc, while the nebula in Orion has a diameter of nure than 

 half a degree, and the great nebulous and stellar structure 

 shown in our plate has a diameter of more than 20 '. 



There is another class of reasoning which enabli's us in 

 a vague and rough way to fix a superior limit for the 

 density of these nebulous masses. If we suppose the 

 Orion nebula to be a uniform sphere of only a third of a 

 degree (20') in diameter, with an average density of only 

 one millionth of our atmospheric air at the sea level, the 

 mass of the nebula would be such that at the di.siaiice of 

 a CenUinri, assuming gravity to act across interstellar 



