5QO 



NA TURE 



[April iS, 1895 



THE SUN'S PLACE IN NATURE} 



IV. 



THE difference in the appearance of spectral lines and 

 flutings having been explained, I now go on to state that 

 the luminosity referred to, as seen in the meteorite experiment 

 was not one of the lines in the spectrum of magnesium, but one of 

 the flutings. I will next throw this on the screen (Fig. 17), and 

 you will at once see the point. Here is the spectrum of magnesium 

 obtained at the lowest temperature at which we can get any light 

 from it at all. We have a fluting, which resembles closely the 

 carbon fluting, but in a different part of the spectrum. We see 

 that its brightest part is coincident with a certain part of the 

 solar spectrum : and it so happens that the position of the line 

 which l>r. Huggins had observed in the nebula; lies very near 

 the same position of the solar spectrum. 



That, then, was one argument out of a great numberjin favour 

 of the view that the luminosity to which the bright line of the 

 nebula was due, might really be produced in the nebulx by 

 collisions of meteorites among themselves, rendering luminous 

 the vapours of magnesium which we knew to be wherever there 

 are meteorites. 



Now, an additional argument for that view was found in the 

 fact that almost every observer, including Dr. Huggins himself, 

 bad stated that as seen in the spectrum of a nebula the line did 

 look somewhat difTerent on one side to what it did on the other, 

 and references were made to its being more degraded on the 

 left-hand side than it was on the right. I h.-id frequently 



light from the nebula of Orion on a powerful horizontal 

 telescope placed in front of a large spectroscope, both rigidly 

 fixed. In order to check the observations as far as possible, I 

 placed in front of the object-glass of the telescope an arrange- 

 ment by which the light from a magnesium wire might enter the 

 slit of the spectroscope at the same lime as the light of the 

 nebula, so that if the light from the nebula and the light from 

 the magnesium wire perfectly agreed in wave-length, we should 

 get one line ; if it differed, we should get two. 



The slit of this spectroscope was exactly in the focus of the 

 ten-inch object-glass, and then the light was passed through 

 four dense prisms, so that we got a consideraole amount of 

 dispersion, and the exact position of the line, whether single or 

 double, was observed. That of course wcs a very much 

 more powerful dispersion than had been employed by Dr. 

 Huggins in his first observations, and muc'i more powerful 

 than had been employed by myself in my frst investigation. 

 But what I wished to do in those first investigations w.-is to 

 understand and to clearly follow the observations which had been 

 made previously by others : if therefore I hid attempted to go 

 over the ground «ith instruments ten times better, giving me 

 ten times finer results than my predecessors had obtained, it 

 would have been the worst possible way togo to work, because 

 it was essential for me to make the necessiry comparisons with 

 the old observations while not exceeding tie instrumental means 

 which had been employed to obtain then. 



The long and short of my various nethods of observation 

 was that they seemed entirely to confirn the idea which I got 

 in the first instance from using telescipes and spectroscopes 

 of very much smaller power. That, lowever, fortunately, for 



Fig. 17. — Spectra of bumine ni.igne^iiim compared with solar spectrum. 

 (1) Sun. (?) Magnesium. 



observed myself that the line representing the chief line of the 

 nebulx was degraded to the left, never to the right, over parts 

 of the nebula of Orion which were more brilliant than the others ; 

 and at the same time that another line — about which more 

 presently — instead of being degi.ided to Ihe left, like this one, 

 was equally eased oflT on both sides (Kig. 18), so that the argument 

 was complete that the appearances presented by this line 

 were not due to any instrumental defect, because, in that case, 

 all the lines would have behaved in the same abnormal manner. 

 Hence then I found myself justified in concluding and sub- 

 sequently stating (I; that the position of the meteoritic dust- 

 line was coincident with the line of the nebulae in the apparatus 

 which I used, and (2) that it resembled it in appearance. 



What I had next to do in the matter was, of course, to carry 

 the thing as far towards the truth as I could. We can never 

 find out the whol^ truth, but it is better to have a part of the 

 truth than none at all. Hence I started a new method of 

 attack, which, you will note, differs very considerably from any- 

 thing you have seen before. I have here a beautiful instrument 

 invented by that eminent I'rcnchman Foucault, called a "sidcro- 

 Stat." The essential part is a plane mirror (Fig. 19) which, when it 

 i» properly adjusted to the sun or moon or any star in any part 

 of the sky, lays hold of a beam of light from it about twelve 

 inchei in diameter, and sends that beam in a horizontal direction 

 due south, and keeps it there ; so that the light falls fairly on 

 the optical apparatus, and we can go on observing it for a 

 i,,n.. i.,„. Next the instrument was adjusted to throw the 



n shorthand nolc«ofa course of Lecture.^ to Working Men 



- of Practical Geology durinK November and December, 



Tov<- t'-ontinucd from pAce 567.) 



NO. 1329, VOL. .si] 



I'iG. 1^. — .\ppearance of principal'J'ics in spectrum of Orion nebula, as 

 observed afVestgatc-on-Sea. 



science, did not satisfy Dr.Iuggins; hevery wisely appealed to 

 the .-\merican astronomei, and I am glad to say that the 

 skilful astronomers of theLick Observatory took up this work 

 with interest, and emplred instruments in the investigation 

 more powerful than an- 1 possessed, thus carrying matters a 

 stage further. There wre leally two distinct bits of work to 

 be done ; first of all, ne wanted the exact position of the 

 line in the nebula;, ani after having got its right position, its 

 origin could be thougl out. We wanted also to see what the 

 real physical appeara^e of the line was, i.e. whether it was 

 most likely a line or fluting. It is not a little curious to 

 note that all the sla nients which had been made suggesting a 

 fluted character of "c line were at once withdrawn when I 

 referred its origin t 'he magnesium fluting. 



The Lick telesc'C 's one of very considerable power indeed, 

 and it is so solidly'U'jt that a very powerful spectroscope can 

 be put on one en of 't and used under almost the best possible 

 conditions for t^ermining the position of lines. .Still the 

 Lick telescope not Ihe best possible telescope to employ for 

 any branch of\«'k connected with nebula;, if thewotk requires 

 a great amour of light, because the longer the Iclcscope, 

 the larger the "gc which the object-glass gives ; for instance, 

 if you are dfipg «'lh a nebula one degree in diameter, if 

 yoiir one degeis written on a circle with a radius of sixty feet, 

 it will be a efy much bigger thing than if on a r.idius of 

 ten feet, scyo" RCt a large image without increasing the 

 light, and erefoie arc spieading your light over a very large 

 area. As^e slit of the spcclroscope is a very small thing, 

 all the li'' «hich is thrown outside Ihe slit is of no use for 

 your spcfoscopic obscrvalicn, so, whatever the size of the 

 spectres pe may tc, you want to deal with the smallest and 



