TEE SEX'S COROXA AXE EIS SPOTS. 



547 



the total eclipse of 177S is rendered doubtful by 

 his reference to an apparent rotatory motion of 

 the coronal rays. He says that about five or six 

 seconds after totality had begun a brilliant lu- 

 minous ring was seen around the dark body of 

 the moon. The ring became brighter as the mid- 

 dle of totality approached. " About the middle 

 of the eclipse, the breadth of the ring was equal 

 to about a sixth of the moon's diameter. There 

 seemed to issue from it a great number of rays 

 of unequal length, which could be discerned to a 

 distance equal to the moon's diameter." Then 

 comes the part of D'Ulloa's description which 

 seems difficult to accept. He says that the coro- 

 na " seemed to be endued with a rapid rotatory 

 motion, which caused it to resemble a firework 

 turning round its centre." The color of the light, 

 he proceeds, " was not uniform throughout the 

 whole breadth of the ring. Toward the margin 

 of the moon's disk it appeared of a reddish hue; 

 then it changed to a pale yellow, and from the 

 middle to the outer border the yellow gradually 

 became fainter, until at length it seemed almost 

 quite white." Setting aside the rays and' their 

 rotation, D'Ulloa's account of the inner corona 

 may be accepted as satisfactory. The height of 

 this ring was, it seems, about 140,000 miles, or 

 twice that of the ring seen last July. As the year 

 1TT9 was one of maximum solar disturbance, 

 there were doubtless many spots in 1778; and 

 the aspect of the corona accorded well with the 

 theory that the corona expands as the number of 

 sun-spots increases. 



We come now to three eclipses which are es- 

 pecially interesting as having been all carefully 

 observed, some observers hawing seen all three — 

 the eclipses, namel}-, of 1S42, 1851, and 18C0. 

 Unfortunately, the eclipses of 1842 and 1851 oc- 

 curred when the sun-spots were neither at their 

 greatest nor at their least degree of frequency. 

 For a maximum of sun-spots occurred in 1837, 

 and a minimum in 1844, so that 1S42 was on 

 what may be called the descending slope of a sun- 

 spot wave, nearer the hollow than the crest, but 

 not very near either. Again, a maximum oc- 

 curred in 1848, and a minimum in 1856, so that 

 1851 was also on the descending slope of a sun- 

 spot wave, rather nearer the crest than the hol- 

 low, but one may fairly say about midway be- 

 tween them. Still it is essential in an inquiry of 

 this sort to consider intermediate cases. We 

 must not only apply the comparcntim ad intellectum 

 instantiarum convenientium, but also the compa- 

 rentia instantiarum secundum magis ac minus. If 

 the existence of great solar disturbances causes 



the corona to be greatly enlarged, as compared 

 with the corona seen when the sun shows no 

 spots, we should expect to find th^e corona mod- 

 erately enlarged only when the sun shows a con- 

 siderable but not the maximum number of spots. 

 And, again, it is conceivable that we may find 

 some noteworthy difference between the aspect 

 of the corona when sun-spots are diminishing in 

 number, and its aspect when they are increasing. 

 This point seems the more to need investigation 

 when we note that the evidence derived from 

 eclipses occurring near the time either of maxi- 

 mum or of minimum solar disturbance has not 

 been altogether satisfactory. It may be that we 

 may find an explanation of the discrepancies we 

 have recognized in some distinction between the 

 state of the corona when spots are increasing 

 and when they are diminishing in number. 



It is noteworthy that several careful observ- 

 ers of the corona in 1S42 believed that they could 

 recognize motion in the coronal rays. Francis 

 Baily compared the appearance of the corona 

 to the flickering light of a gas-illumination. 0. 

 Struve also was much struck by the appearance 

 of violent agitation in the light of the ring. It 

 seems probable that the appearance was due to 

 movements in that part of our atmosphere 

 through which the corona was observed. The 

 extent of the corona was variously estimated by 

 different observers. Petit, at Montpellier, as- 

 signed to it a breadth corresponding to a height 

 of about 200,000 miles; Baily a height of about 

 500,000 miles ; and O. Struve a height of more 

 than 800,000 miles. The last-named observer 

 also recognized luminous expansions extending 

 fully four degrees (corresponding to nearly 7,000,- 

 000 miles) from the sun. Picozzi, at Milan, no- 

 ticed two jets of light, which were seen also by 

 observers in France. Kays also were seen by 

 Mauvais at Perpignan, and by Baily at Pavia. 

 But Airy, observing the corona from the Superga, 

 could see no radiation ; he says, " Although a 

 slight radiation might have been perceptible, it 

 was not sufficiently intense to affect in a sensible 

 degree the annular structure by which the lu- 

 minous appearance was plainly distinguished." 

 These varieties in the aspect of the corona were 

 doubtless due -to varieties in the condition of the 

 atmosphere through which the corona was seen. 

 Now, it cannot be questioned that, so far as ex- 

 tension is concerned, the corona seen in 1S42 

 was one which, if the theory we are considering 

 were sound, we should expect to see near the time 

 of maximum rather than of minimum solar dis- 

 turbance. On the other hand, in brightness the 



