July 1, 1892.] 



KNOWLEDGE 



131 



The accompanying diagrams, in which an attempt is made 

 to repi'Bsent the manner of spot distribution at different 

 periods of the cycle, will perhaps set forth these changes 

 more clearly than much description can do. 



So strongly marked a relation must evidently be given 

 great weight in any theory which we form as to the origin 

 and nature of sunspots. Unfortunately, I cannot claim 

 to have framed any such theory myself, and the only point 

 1 would urge is that these various relations — the sudden 

 appearance of the spots of a new cycle in high latitudes, 

 the persistent decline in latitude of the general spotted 

 area as the cycle progresses, and the drift in latitude of 

 individual groups — seem to me absolutely fatal to the idea, 

 once popular, that the secret of solar disturbances lies 

 without the sun ; in the relative positions of the planets, 

 for example, or in the fall of meteorites. 



If this purely negative conclusion is felt to be dis- 

 appointing, I cannot help it. I feel no mission myself to 

 explain everything, but rather prefer to state the facts as I 

 know them for others to explain. Yet it is no light 

 matter in our progress towards truer theories that we 

 should get rid of the false ones. 



I think, therefore, we may settle ourselves that the 

 secret of the sunspot problem lies below the solar surface, 

 not above it ; and the well-known fact, that outbreaks often 

 recur in the same districts after considerable intervals of 

 time, appears to me to strongly confirm it. 



There is one other circumstance, in connection with this 

 decline in latitude as the spot-cycle progresses, to which I 

 should like to allude. Just as the increase in the number and 

 size of spots after minimum does not proceed regularly, 

 but by waves, one fortnight being rich in spots, and the 

 next poor, and so on, oscillating to and fro, but the days 

 without spots becoming ever fewer, and the days with 

 spots both more numerous and richer, so it is with the 

 decline in latitude. It proceeds in waves. Every fourth or 

 fifth rotation there will be an effort to I'each a higher 

 level, a lift of one or two degrees, and then a gradual 

 slipping back until a fresh effort brings another small Uft, 

 but a weaker one than the last. And so the cycle goes 

 on ; the decline is continual on the whole, but, is broken 

 and interrupted by these frequent little struggles to get 

 back to a higher plane. One could almost fancy one saw 

 a living being trying to maintain a losing battle with ever- 

 declining strength. No doubt these minor oscillations 

 proceed from a cause similar to that which gives rise to the 

 gi-eat lift in latitude at the beginning of a new cycle, and 

 it is in the attentive study of the laws according to which 

 these changes occur that we may hope to unravel their 

 meaning, and in so doing add to our knowledge of the 

 constitution of the sun. 



WHAT IS A NEBULA? 



By A. C. Raxyakd. 



THE plate which illustrates this paper has been made 

 from a photograph kindly sent me by Dr. Max 

 Wolf, of Heidelberg. It represents a region of the 

 Milky Way not very distant from the regions repre- 

 sented in the plates published with the October 

 and December numbers of Knowledge for last year. 



The bright star near the centre of the plate is /3 Cygni, 

 a beautifully coloured double star well known to the pos- 

 sessors of small telescopes as a very easy test object. It is 

 not shown as double in the plate because the photographic 

 trace left by the two stars is a patch, or disc, more than a 

 tenth of an inch in diameter, which corresponds to some 

 seven or eight minutes of arc upon the heavens, while the 

 two stars, though they form a pair which may be easily 



split by a good opera glass, are only 34" apart — that is, on 

 the scale of the plate their centres would be separated by 

 a distance of only about ^ l^jth of an inch. This minute 

 separation, though not recognizable on the photograph, 

 corresponds to a real distance in space which is so great 

 that during the time in which man has been observing 

 them no motion of one star about the other has been noted, 

 though their colours would lead us to conclude that they 

 are really associated. The larger star, which is of the 

 third magnitude, shines with a golden yeUow light, while 

 the smaller star of the seventh magnitude is a beautiful 

 azure blue, thus conforming to the general law that in 

 coloured binaries the smaller star is always bluer than the 

 larger one. 



This plate of Dr. Max Wolf's, although it was exposed 

 for twelve hours, does not show as much nebulosity as the 

 plates of the regions around a Cygni and g Cygni, pub- 

 lished in the October and December numbers of Kno^vledge ; 

 but especially in the lower half of the plate there is a very 

 distinct background of nebulosity, interrupted here and 

 there by dark channels and dark prominence-like forms, 

 associated with lines of small stars. 



The nebulous background of the Milky Way is too faint 

 to allow of its hght being analysed with the spectroscope, 

 but its whitish colour, as seen with the naked eye, seems 

 to indicate that it does not belong to the type of nebula 

 which gives bright lines or a gaseous spectrum. 



About half the nebulaj that have been examined spectro- 

 scopically give a spectrum in which six or seven lines are 

 fairly conspicuous. The three brightest of these lines are 

 situated in the green, ' and they give to this class of 

 nebula a verj- distinct bottle-green tint that enables an 

 observer with a large telescope to recognise one of these 

 gaseous nebuhe at sight as differing from a white nebula, 

 such as the nebula in Andromeda, which gives a continuous 

 spectrum unmarked by any well-recognizable lines or 

 bands. 



The faint continuous spectrum of the white nebuhe is 

 not crossed by any dark absorption lines such as we see in 

 the spectra of the stars. If the light of these nebulie were 

 due to thinly scattered faint stars, too small to be 

 individuaUy visible, we might expect their combined light 

 to give a faint spectrum crossed by dark absorption lines 

 common to the spectra of the small stars. That such 

 absorption lines are not visible in the spectra of the white 

 nebuhe is prima fdcic evidence that they do not consist of 

 sparsely distributed bodies similar in constitution to the 

 brighter stars. 



It has been assumed that the greenish nebulae which 

 give bright line spectra are masses of incandescent or 

 glowing gas ; but such masses of gas, if quiescent in space, 

 would be cooler on their outsides, and the outer cold layers 

 would, according to the theory of exchanges, absorb the 

 radiations given out by the glowing gas within. The 

 bright line spectrum of these uebulie seems always to be 

 accompanied by a more or less faint background of 

 continuous spectrum, such as would be given out by glow- 

 ing solid or liquid particles, or by gas under pressure. 

 Reasoning from first principles, it seems probable that a 

 quiescent mass of hot gas cooling iu space would give out 

 a continuous spectrum from its lower regions, where it is 

 under pressure, so that the free paths of its molecules are 

 relatively short, and this faint continuous spectrum would, 

 it seems probable, be channelled by dark lines in places 



* All the gaseous nebuliB give approximately the same tjpe of 

 spectrum. The brightest line is situated in the green region, at waye- 

 length 5O0 4 ; the next in brightness is al.so in the green, at wave- 

 length 49.5-8 ; anil the next in brightness is the well-known bluish- 

 green F line of the hydrogen spectrum situated at wave-length 486'1. 



