April i8, 1895] 



NATURE 



591 



brightest possible image in order to get the best use out of 

 ■your spectroscope, and ihat cannot be done with a long focus 

 telescope. However, the important question for the American 

 observers was to determine the exact position of the line ; 

 and we have lately been given some very interesting results. 



Fig. 20 represents the way in which Dr. Keeler puts 

 his last result. The upper part is a representation of the solar 

 spectrum ; the numbers represent wave-lengths on Rowland's 

 scale. According to his latest value the wave-length of the 

 nebular line 155007-05. He also shows in relation to it the lines 

 of nitrogen as well as the fluting of magnesium, and you see at 

 once that, although according to this drawing the magnesium 

 does not quite correspond with the line of the nebul.t, it is 

 very much nearer to it than is either the line of lead or 

 the lines of nitrogen. The publication of this result 

 necessitated a fresh investigation, to see what the exact 

 facts were when we no longer compared the nebula with 

 magnesium, but compared the magnesium with the' solar 

 spectrum, and therefore sought the true position in which 

 to place the magnesium line in relation to the solar spec- 

 trum. 



Here is the result. Vou will see that there is a very small 

 dflference between the position of the magnesium fluting and 

 he nebular line. In short, the more the work done on this 



■ Fig. 19. — Showing object-glass ofyiorizontal telescope used with siderostat 



question 'the more and more coincident have these lines be- 

 come, and there are some iconsiderations which have not yet 

 been taken into account. 



I have referred to this point somewhat at length, although 

 the coincidence with the fluting of magnesium is not funda- 

 mental for the establishment of the view which even Dr. 

 Huggins has now accepted, in the way I have already 

 Stated. 



Now, what was the chemical constitution of the nebula stated 

 to be as a result of the first spectroscopic observations ? Dr. 

 Huggins, in his paper of 1865, to which I have already 

 referred, was of opinion that the chief line was due to 

 nitrogen. 



Here are two tubes, one containing hydrogen, the other 

 nitrogen. You will see at once that these two gases, when I 

 set them glowing by the passage of an electric current, are very 

 different in colour ; we get in one an excess of red light, in the 

 other we have a purple tinge. When nitrogen is observed by 

 means of a spectroscope, a double line is seen very nearly 

 coincident with the line of the nebul.x-. Dr. Huggins thought 

 that one of the constituent lines was exactly coincident with it, 

 and because there was apparently no line whatever correspond- 

 ing with the other, he thought also that the nitrogen might not be 



NO. 1329, VOL. 51] 



nitrogen like that with which we are familiar, but an unknow^a 

 form of it. There was no doubt from the beginning that 

 another line was a line of hydrogen, although there was some 

 slight doubt as to whether the hydrogen in the nebulae behaved 

 exactly like hydrogen on the earth. 



Nobody believes in the nitrogen constituent of the nebulae 

 now; and I presume Dr. Huggins has withdrawn in fact, if 

 not in words, his statement concerning the coincidence, 

 for in his address as President of the British Associa- 

 tion, in which, as I have already stated, he withdrew his pub- 

 lished statement as to the position of the nebulx a-nong the 

 various bodies that people space, he remarks, "the progress of 

 science has been greatly retarded by resting important conclu- 

 sions upon the apparent coincidence of single lines in spectro- 

 scopes of very small resolving power," an apolo^a of which 

 every one will see the propriety, for you will gather from Dr. 

 Keeler's diagram that the nearest nitrogen line is three limes 

 further removed from his position of the nebula line than is the 

 magnesium fluting. I trust I shall not be thought to be 

 exceeding the bounds of decorous criticism when I remark that 

 while Dr. Huggins has referred to the inaccuracy of my 

 work in relation to this line, which is apparently indicated by 

 Dr. Keeler's results, he has never pointed out the three times 

 greater inaccuracy of his own. 



In order to give you an idea of the 

 relative accuracy which all these refer- 

 ences to wavelength indicate, let us sup- 

 pose that we are trying to define the 

 position of a place in London on an E. 

 and W. line running through Charing 

 Cross, and then you will see exactly how 

 matters stand. Assuming Dr. Keeler's 

 value to be absolutely true — and I expect 

 it is as near the truth as we are likely to 

 get for some lime — we will suppose that 

 it represents the nebular line situated on 

 the statue of King Charles at Charing 

 Cross. When Mr. Huggins first raea- 

 .sured it, he brought it to the East India 

 Docks ; his next attempt brought it to 

 Hammersmith. Dr. Keeler's first obser- 

 vation brought it to .\lbert Gate ; his 

 next, in 1891, brought it to S:. James's 

 Palace. Subsequent work at Kensington, 

 not yet completed, has brought it nearer 

 still. 



There is another argument in favour of 

 the now accepted view which may be 

 gathered from a careful examination of the 

 forms of these different nebulas, and by 

 endeavouring to reason out from the form 

 what the actual conditions at work may be. 

 One of the most wonderful spiral nebulae 

 in the heavens is that in the constel- 

 lation of Canes Vanatici, which has 

 been photographed by Dr. Roberts 

 (Fig. 7, p. 397). This is a nebula which 

 we look down upon ; we see it in plan ; we are, so to speak, at 

 the pole of the system, so that it is not foreshortened. 



There is no question about the wonderful spirals being connected 

 with the central condensation and stretching away from it, and 

 the point which I made with regard to the one in Ursa Major 

 is even more decided here, when I call your attention to these 

 points of condensation right along one of the spiral branches, 

 and when you get the possible intrusion of two spirals one on 

 the other you see a confused mass of light. Now, if we 

 imagine ourselves dealing there with a mass of pure gas, 

 whether it is hydrogen or nitrogen or ammonia — that is, a 

 combination of both — or any other, i; would be extremely 

 diflicult to see why there should be any change in tempera- 

 ture in diffijrent parts of that mass ; but the moment you 

 assume that you are dealing with cool materials — meteoritic 

 dust — you will see that such a picture as this is important 

 to u.s, for the reason not that it shows us what is there, 

 but because it shows us what is going on there. These 

 bright spots do not represent the presence of matter, and 

 the dark ones the absence of matter : bat these brighter 

 portions represent the stream lines where collision is possible 

 — the intervals those regions where collisions are less likely, 

 and you will see from the very configuration of this 



