May 10, 1888] 



NATURE 



33 



the spectra are known to exist, should show that all the 

 differences can be accounted for in the main by differ- 

 ences in the amount of interspace ; that is to say, by a 

 difference between the relative areas of space and 

 meteorite in a section of the swarm at right angles to the 

 line of sight. I say in the main, because subsequent in- 

 quiry may indicate that we should expect to find minor 

 differences brought about by the beginnings of condensa- 

 tion in large as opposed to small swarms, and also by the 

 actual or apparent magnitudes of the swarms varying 

 their brilliancy, thus enabling a more minute study to be 

 made of the same stage of heat in one swarm than in 

 another. 



How minor differences may arise will be at once seen 

 when we consider the conditions of observation. 



The apparent point of light generally seen is on my 

 view produced not by a mass of vapour of more or less 

 regular outline and structure,but by a swarm of meteorites 

 perhaps with more than one point of condensation. 



An equal amount of light received from the body may be 

 produced by any stage, or number of nuclei, of condensa- 

 tion ; and with any differences of area between the more 

 luminous centre and the outliers of the swarm. 



All these conditions producing light of very different 

 qualities are integrated in the image on the slit of the 

 spectroscope. 



I have said " generally seen," because it has been long 

 known that many of the objects I am now discussing are 

 variable, as well as red, and that at the minimum they 

 are not always seen as sharp points of light 1 but have 

 been described as hazy. 



The severe nature of the tests at our disposal will be 

 recognized when we inquire what must follow from the 

 variation of the spacing. Thus, as the spacing is 

 reduced — 



I. The temperature must increas8. 



a. Vapours produced at the lowest temperatures 

 will be the first to appear. 



£. The spectrum of each substance must vary with 

 the quantity of vapour produced as the 

 temperature increases, and the new absorp- 

 tions produced must be the same and must 

 follow in the same order as those observed in 

 laboratory experiments. 



II. The carbon spectrum must first get more intense 

 and then diminish afterwards as the spaces, now smaller, 

 are occupied by vapours of other substances. 



a. The longest spectrum will be that produced by 



mean spacing. 

 0. The masking of the dark bands by the bright 



ones must vary, and must be reduced as the 



mean spacing is reduced. 



III. The continuous spectrum of the meteorites must 

 increase. 



a. There will be a gradually-increasing dimming 

 of the absorption-bands from this cause. 



j3. This dimming will be entirely independent of 

 the width of the band. 



IV. The spectrum must gradually get richer in absorp- 

 tion-bands. 



a. Those produced at the lowest temperatures will 



be relatively widest first. 

 fi. Those produced at the highest temperatures will 



be relatively widest last. 

 y. They must all finally thin. 



These necessary conditions, then, having to be fulfilled, 

 I now proceed to discuss M. DuneVs individual observa- 



1 Hind first no'.ic 1 t'lls in 1851. 

 Populaire." 



Quoted by Arago, "Astronomie 



tions. I shall show subsequently that there are, in all 

 probability, other bodies besides those he has observed 

 which really belong to this group. 



II. Discussion of Dun£r's Individual Cbservations. 

 Consideration of the Extreme Conditions of Spacing. 



Ceteris paribus, when the interspaces are largest we 

 should have a preponderance of the radiation of carbon, 

 so far as quantity goes. The bands will be wide and 

 pale, the complete radiation will not yet be developed ; 

 a minimum of metallic absorption phenomena— that is, 

 only the flutings of magnesium (8 and 7), the first fluting 

 of manganese (3), and the first fluting of iron (2) ; but the 

 great width of the bright band at 517 will mask band 8. 



When the interspaces are least, the radiation of 

 carbon should give place to the absorption phenomena 

 due to the presence of those metallic vapours produced 

 at the highest temperature at which a swarm can exist 

 as such; the bright flutings of carbon should be dimin- 

 ished, and the true absorption flutings of Mg, Fe, Mn, Pb, 

 and the band of Ba, should be enhanced in intensity. 



There will be an inversion between the radiation and 

 absorption. 



The highest intensity of the absorption phenomena will 

 be indicated by the strengthening of" the bands 2, 3, 4, 5, 

 and 6 ; and the appearance of the other flutings and 

 bands specially recorded in a Ononis. The bands 7 and 

 8 will disappear as they are special to a low temperature, 

 and will give way to the absorption of manganese, iron, 

 b, &c. 



This inversion, to deal with it in its broadest aspect 

 should give us at the beginning 7 strong, and 2, 3 weak, 

 and at the end 7 and 8 weak, and 2, 3 strong. 



The first stage, representing almost a cometic condition 

 of the swarm before condensation has begun, has been 

 observed in Nos. 3, 1 23, 24, 25, 36, 68, 72, 81, 118, 247, 

 249. There is a very large number of similar instances 

 to be found in the observations. The above are only 

 given as examples. 



The last stage, before all the bands fade away entirely, 

 has been observed in Nos. 1, 2, 26, 32, 33, 38, 40, 61, 64, 

 69, 71, 75, 77, 82, 96, 101, 116. As before, these are only 

 given as instances. 



It is natural that these extreme points along the line of 

 evolution represented in the bodies under consider- 

 ation should form, as I think they do, the two most 

 contrasted distinctions recorded by Dune"r — that is, re- 

 corded in the greatest number of cases. 



Origin of the Discontinuous Spectrum. 



I have already shown that when the meteorites are 

 wide apart, though not at their widest, and there is no 

 very marked condensation, the spectrum will extend 

 farther into the blue, and therefore the flutings in the blue 

 will be quite bright ; in fact, under this condition the chief 

 light in this part of the spectrum, almost indeed the only 

 light, will come from the bright carbon. Under this same 

 condition the temperature of the meteorites will not be very 

 high, there will therefore be little continuous spectrum to 

 be absorbed in the red and yellow. Hence we shall have 

 discontinuity from one end of the spectrum to the other. 

 This has also been recorded, and in fact it is the condi- 

 tion which gives us almost the most beautiful examples of 

 the class (196, a Herculis, 141, 172, 229). 



The defect of continuous light in the blue in this class, 

 after condensation has commenced and the carbon flutings 

 are beginning to disappear, arises from defect of radiation 

 of the meteorites, and hence in all fully-developed swarms 

 the spectrum is not seen far into the blue for the reason 

 that the vapours round each meteorite are at a tempera- 



1 The references are to the numbers of the stars in Duner's catalogue. 



