ASTRONOMICAL PHENOMENA AND PROGRESS. 



39 



grew steadily shorter (not less brilliant what remained 

 of it), until finally its last spark vanished, the C line 

 became exactly like its neighbors, and the contact was 

 effected. 



The observation was as easy and definite as ^that 

 of the transit of a moderately slow star. I am confi- 

 dent the observation may bo relied upon within a frac- 

 tion of a second, although it was from 5 to 15 seconds 

 earlier than the time assigned by any of the other ob- 

 servers. I am informed by Professor Mayer, however, 

 who had charge of the photographic operations of our 

 party, that it agrees within one-third of a second with 

 the time deduced from a preliminary measurement 

 of a photograph taken about 15 seconds after the con- 

 tact was announced. 



"With an instrument of sufiicient dispersive power, 

 the slit might be opened somewhat widely, and placed 

 tangent tol;he sun's limb. In this case a slight error 

 in the estimated point of contact would not interfere 

 with the accuracy of the observation. 



I wish to call attention to the applicability of this 

 method at the coming transits or Venus. It is not 

 possible, perhaps, to predict just how great will be 

 the effect of her atmosphere: but it is difficult to see 

 in what respects this method will suffer from it more 

 than any other. It certainly presents this great ad- 

 vantage, that the observer will perceive and watch 

 the planet's approach long before the instant to be 

 observed, and thus have all the benefit of prepara- 

 tion. 



It seems likely also that the instant of internal con- 

 tact will be more easily seized with the spectroscope 

 than with any other instrument. Instead of the rup- 

 ture of a black ligament, it ought to show the sudden 

 formation of a brilliant line running the whole length 

 of a before dusky spectrum, a phenomenon much 

 more striking than the other. 



While the moon was advancing upon the sun, spe- 

 cial attention was paid to the appearance of the spec- 

 trum lines near her limb. They came up to the 

 edge perfectly square and straight, even when the 

 limb made an angle of only 5 or 6 with the slit ; 

 and the longitudinal line of demarcation, before re- 

 ferred to, between the brilliant and dusky portions 

 of the spectrum, was hard and sharp, in striking con- 

 trast with the effect of the sun's limb, which, under 

 similar circumstances, always gives a boundary more 

 or less hazy arid indefinite, and this to a degree con- 

 tinually changing from minute to minute. This con- 

 trast was beautifully exhibited a few seconds before 

 the totality, when the limbs of both sun and moon 

 were on the spectrum together, the width of the visi- 

 ble portion of the sun having become less than the 

 length of the slit. It was at first thought that this 

 appearance was decisive against the existence of a 

 lunar atmosphere, however rare ; but a little consider- 

 ation shows that on the other hand it is, if any thing, 

 favorable, being a simple consequence of that bright- 

 ening of the sun's disk near the moon's limb which 

 is so beautifully evident upon the photographs ; and 

 which is most easily accounted for by admitting a 

 slight refraction suffered by that portion of the sun- 

 light which grazes the moon. Possibly, however, it 

 ma y yet be explained as a case of simple inflection 

 of light. 



Before the eclipse began, the existence of promi- 

 nences on the limb of the sun had been ascertained 

 in the following positions (reckoning from the north 

 point through the east). A large but faint one near 

 + 90, a small but bright one at +146 (the photo- 

 graphs show two here), a long low one at -70 , very 

 near the point of first contact, and an enormous and 

 very bright one at -130, with several others of small 

 elevation, but considerable length, on different parts 

 of the limb. 



In his examination of the prominences Pro- 

 fessor Young observed 9 bright lines, the most 

 remarkable of which were 0. F. and an orange 

 line. 



A faint continuous spectrum, without any traces 

 of dark lines in it, was also visible, evidently due to 

 the corona. Its light, tested by a tourmaline applied 

 next the eye, proved to be very strongly polarized in 

 a plane passing through the centre of the sun. I am 

 not sure, however, but that this polarization, as sug- 

 gested by Professor Pickering, may have been pro- 

 duced by the successive relractions through the 

 prisms. This explanation at once removes the diffi- 

 culty otherwise arising from the absence of dark 

 lines. 



My observations decide nothing as to specific dif- 

 ferences between the different protuberances^ since, 

 from the smallness of my field of view, I was obliged 

 to observe a portion of the spectrum on one of the 

 prominences and the rest on another. 



In conclusion,. Professor Young remarks that, 

 at present, it seems pretty likely that the 

 spectra of the corona and the aurora borealis 

 are identical, with only such differences as we 

 might naturally expect, and that very probably 

 the identity extends to the essential nature of 

 the phenomena themselves. With reference to 

 the iron line observed in the spectrum, he says 

 it will he of interest to inquire whether we are 

 to admit the existence of iron vapors in and 

 above our atmosphere, or whether in the spec- 

 trum of iron this line owes its presence to some 

 foreign substance probably some occluded 

 gas, as yet unknown, and perhaps standing in 

 relation to the magnetic powers of that metal. 



Professor Edward 0. Pickering, who was also 

 with the Nautical Almanac party at Burling- 

 ton, sends the result of his observations to 

 the Philosophical Magazine. He says that, in 

 examining the photographs taken by the party, 

 it was noticed that, while the light diminished 

 near the edge of the sun, the moon's limb was 

 very distinct, and that there was a marked 

 increase in the light of the parts nearest it. 

 The best explanation of this phenomenon the 

 author thinks is to assume the presence of a 

 lunar atmosphere. The corona would then 

 be caused by refraction of light reaching the ob- 

 server from parts of the sun already eclipsed. 

 This hypothesis is strengthened by other ob- 

 servations. The protuberances have often 

 seemed to indent the moon's edge, an appear- 

 ance usually ascribed to irradiation. An at- 

 mosphere of rapidly-increasing density might 

 produce this effect by reflection, and of course 

 would not influence the corona if it was caused 

 by refraction. The principal reason for sup- 

 posing the corona a portion of the sun is, that, 

 during totality, it does not appear to move 

 with the moon, but remains concentric with 

 the sun, or, more properly, is brightest where 

 the sun's edge is nearest. Many of the photo- 

 graphs show this very well, the difference on 

 the two opposite sides of the moon being very 

 marked. This effect could be explained equal- 

 ly well by supposing the corona caused by 

 refraction. For the centres of the sun and 

 moon never differ during totality by more 

 than half a digit, while the breadth of the 

 corona is sometimes several times as much; 

 so that merely covering a small portion of it 

 would not produce a greater diminution of 



