82 



NATURE 



\^Nov. 24. 1887 



fore, that we have here again strong ground for rejecting the view 

 that the lines seen in "stars" at 540 and 580 are due to cool 

 H, for since hydrogen is common to both nebulse and stars, there 

 is no rea-on why structure lines should occur in "stars "any 

 more than in nebulae. 



Another ground for rejecting cool hydrogen as the origin of 

 any of the lines in "stars" is that the structure-spectrum of 

 hydrogen is only seen in confined glows, which is just the condi- 

 tion which cannot occur in space. 



At the same time, the apparent coincidences of many meteorite 

 lines with structure lines of hydrogen greatly increases the 

 difficulties of laboratory work ; in fact, the structure-spectrum of 

 hydrogen is to observations of meteorite glows in the laboratory 

 what continuous spectrum is to observations of bright lines in 

 stars. 



If it be agreed that we are not dealing with cool hydrogen, 

 then it will follow that the only difference between celestial 

 bodies with bright lines in their spectra comes from no differ-: 

 ence of origin or chemical constitution, but from a difference 

 of temperature. 



At one point in these researches I was under the impression 

 that the differences in the systems of bright lines seen in the 

 nebulae and the bright-line stars might arise from a preponder- 

 ance of irons or stones in the swarms. But I was led to abandon 

 this idea, not only by the observation of the meteoritic glows, 

 but by the consideration that even telescopically the "stars" 

 in question are more condensed than the nebulce. 



The spectrum of the nebulae, except in some cases, is associated 

 with a certain amount of continuous spectrum, and meteorites 

 glowing at a low temperature would be competent to give the 

 continuous spectrum with its highest intensity in the yellow pai t 

 of the spectrum ; so that in this way we should understand 

 that lines due to any gas or vapour in that part would be very 

 much more likely to escape record than those in the part of the 

 spectrum which the continuous spectrum hardly reaches. The 

 general absence, however, of bright lines of metallic vapours, 

 except 495 and 500, and of the bright lines of hydrogen, evidently 

 justifies the conclusion that we are here in presence of those 

 bodies in celestial space, connected with which the temperature 

 and the electrical excitation are at the minimum, and it is very 

 remarkable how the lines seen in a Geissler tube under the con- 

 ditions stated, when either magnesium, or olivine, or other 

 meteoric constituents are made to glow, should appear, one may 

 almost say, indiscriminately among the orders of bodies in the 

 heavens which up to the present time have been regarded as 

 so utterly different in plan and structure as stars and nebulae. 



The records of purely continuous spectra in the case of many 

 nebulae, as, for example, the Great Nebula in Andromeda, is 

 in all probability an indication of our inability to observe them 

 properly. For a nebula to give aperfecily continuous spectrum, 

 it is evident that the component meteorites must be incandes- 

 cent, but still at a lower temperature than that required to give 

 bright lines. Now, the Mg line 500 is seen in some of the 

 faintest nebulae, where there is little or no continuous spectrum, 

 and it therefore seems likely that these are at a lower tempera- 

 ture than the nebulas said to give perfectly continuous spectra. 

 This being so, it is difficult to believe that other lines, which 

 require a somewhat higher tem'perature for their existence than 

 the line at 500, do not become visible at this increa-ed 

 temperature. 



There can be little doubt that when our ia^trumental ap- 

 pliances and observing conditions become more perfect it will 

 be found that the so-calkd continuous spectra are really discon- 

 tinuous. There is, indeed, an element of doubt as regards some 

 of the existing observations ; thus, the spectrum of the com 

 panion to the Great Nebula in Andromeda appears to end 

 abruptly in the orange, and throughout its length is not uni- 

 form, but is evidently crossed by lines of alsoiption or by bright 

 lines (Huggins, Phil. Trans, vol. cliv. p. 441). 



Again, the Great Nebula in Andromeda is generally regarded 

 as having a continuous spectrum pure and simple, but an 

 observer at Yale College (name not stated) has observed three 

 bright lines in its spectrum {Observa/oiy, vol. viii. p. 385). The 

 lines are — the F line of hydrogen, and two other lines at wave- 

 lengths 53f2'5 and 5594 'o. The latter two lines are mentioned 

 by the same observer as bright lines in 7 Cassiopeiae and /3 Lyrce, 

 and are recorded by Sherman {Astj-. Nach., No. 2591) ns bright 

 lines in these stars and in Nova Andromeda;. No other observa- 

 tions with which I am acquainted give these two lines in 7 

 Cassiopeia; or /3 Lyrae, but Maunder {Monthly Notices, vol. xlvi. 



p. 20) gives them as two of the lines seen in Nova Andromedae. 

 It is possible, therefore, that the two lines in question, in the 

 Yale College observation, had their origin in Nova Andromedje ; 

 at all events there is no evidence to show that they are visible in 

 the Great Nebula of Andromeda under normal conditions. 



It is not impossilile that the lines at 540 and 580 may be 

 eventually traced in some of the brightest nebulae, since these 

 are apparently the lines next in order, as regards temperature, 

 to the Mg line 500. 



It is right that I should here point out that some observers 

 of bright lines in these so-called stars have recorded a line in 

 the yellow which they affirm to be in the position of D3 ; while 

 on the other hand, in my experiments on meteorites, whether in 

 the glow or in the air, I have seen no line occupying this position. 



I trust that some observer withgreater optical means will think 

 it worth his time to make a special inquiry on this point. The 

 arguments against this line indicating the spectrum of the so-called 

 helium are absolutely overwhelming. The helium line so far has 

 only been seen in the very hottest part of the sun which we can 

 get at. It is there associated with b and with lines of iron which 

 require the largest coil and the largest jar to bring them out, 

 whereas it is stated to have been observed in stars where the 

 absence of iron lines and of b shows that the temperature is very 

 low. Further no trace of it was seen in Nova Cygni, and it has 

 even been recorded in a spectrum in which C was absent. 



It is even possible that the line in question merely occupies the 

 position of D3 by reason of the displacement of D by motion of 

 the "stars " in the line of sight. On this point no information is 

 at hand regarding any reference spectrum employed. If, how- 

 ever, it should eventually be established that the line is really Dj, 

 which probat^ly represents a fine form of hydrogen, it can only 

 be suggested that the degree of fineness which i; brought about 

 by temperature in the case of the sun is brought about in the 

 spaces between meteorites by extreme tenuity. 



The Case of Nova Cygni. 



The case of Nova Cygni is being discussed, and it appears 

 likely that this "star" pa-sed through all the stages of tempera- 

 ture represented by "stars" with bright lines, comets, and 

 nebula'. In the initial stage, the principal lines recorded were 

 those of hydrogen, cool magnesium, and sodium. At a later 

 date, in addition to these, lines apparently indicating hotter 

 magnesium and carbon were observed. On the date of its 

 highest temperature (December 8, 1876) the lines observed by 

 Vogel indicate II, Na, Mg, C, Fe, Mo, and Ba, the "star" 

 having then, it would appear from the discussion so far as it has 

 yet gone, approached the condition of the great comet of 1882 

 r.-t perihelion. The Fe, Ba, C, and Na gradually disappeared, 

 then the hydrogen followed, and the last stage of all was that in 

 which Mg (500) appeared alone, as in the comets of 1866-67 

 and in nebula;. The complete discussion, however, must be 

 reserved for a future communication. It is sufficient to say here 

 that it is very probable that all the spectroscopic phenomena of 

 Nova Cygni will admit of explanation on the supposition that 

 it was produced by the collision of two swarms of meteorites. 

 The outliers were fiist engaged, and at the maximum the denser 

 parts of the swarm. 



Difficulties connected with the Discussion. 



An inspection of the maps, on which are shown all the ob- 

 servations already made upon bright lines recorc'ed in the spectra 

 of celestial bodies, will indicate at first sight an apparent variation 

 of the portions of the lines greater than might have been ex- 

 pected. This, however, I think will vanish on the consideration 

 of the whole question ; and for n.y part certainly all the exami- 

 nations which I have been able to make have led me to the 

 conclusion that the various observations have been far better than 

 it was almost possible to hope for when the great difficulties of 

 the observatio.is themselves are considered. 



When it is remembered that, in order to get a determination 

 of the position of a bright line, comparison-spectra and prisms 

 are neeeled, and that, from mechanical considerations alone, 

 the application of the e aids to research is very frequently 

 attended with difficulties and uncertainties ; and further, when 

 we consider that many of the observations have been neces- 

 sarily made without these aids ; the striking coincidences on 

 the maps become of very much greater importance than the 

 slight variations seen between the positions of the same line 

 recorded by different observers in the same star. 



