August 27, 1896] 



NA TURE 



395 



THE ECLIPSE OF THE SUN. 

 Kio ISLANii, Bras Havn, 



Thursday, Aiif^ust 6, 1896. 



BEFi^RE I attempt to give an account of what we 

 have done here and of the local conditions generally, 

 it may be well to state what, in my opinion at all events, 

 is the most important work to be done at eclipses in the 

 present state of our knowledge. 



In looking back along the eclipse records, say till 1870, 

 it is not a little surprising to note how the attack has 

 varied in the importance attributed to certain of the 

 inquiries ; and how often it has happened that the chief 

 scientific result secured at any eclipse was hardly dreamt 

 of by the organisers of the expeditions. But when there 

 has been this notable divergence between anticipation and 

 actual result, the work done has proved of the greatest 

 advantage to science. 1 shall not be sorry, therefore, if the 

 following anticipations fail to include the most important 

 advances made during the coming eclipse. 



In the first place I think the records already obtained 

 by large scale prismatic cameras have shown to every- 

 body that these instruments are the most important ones 

 we can employ on an eclipsed sun. They not only give 

 us a complete chemical record, on a scale hitherto 

 undreamt of, but they give us the positions and forms of 

 the prominences far better than they have ever been 

 obtained before. Nor is this all, they enable us to study 

 under new conditions some of the conclusions arrived at 

 m previous eclipses, and give us a means of inquiring into 

 the possible origins of some of the phenomena already 

 recorded by slit spectroscopes. 



It is now more than a quarter of a century since 

 bright lines were recorded in the spectrum of the dark 

 moon. There could, of course, never have been any 

 doubt that this was due to chromospheric glare in our 

 atmosphere ; but the moment this was conceded, the more 

 difficult it became to determine the exact height of the 

 solar envelopes, for if there was glare over the dark moon, 

 how high might it not extend over the prominences ? 



Now one of the important points about the prismatic 

 camera is that it is quite impossible for it to treat such a 

 general glare as this in the same way it does any local 

 illumination ; as a result of this property any effect due 

 to general glare which can be recorded by a slit spectro- 

 scope cannothi recorded by the prismatic camera, and so, 

 roughly speaking, a comparison of the two records may 

 be safely trusted to eliminate the effects of such glare. 



It will be generally recognised that this is an important 

 service to render ; but there is another which, from the 

 chemical point of view, is more important still — it enables 

 us to localise the origins of the various radiations which 

 build up the spectrum of the sun's surroundings, whether 

 they be high or low. 



For the first time in 1893 the corona was photographed 

 as a ring by means of the prismatic camera in " 1474" 

 light, and apparently associated with it were other rings 

 in the ultra-violet. The 1474 ring was best shown in the 

 Brazilian photographs taken by Mr. Shackleton, but the 

 others in the series taken by Mr. Fowler in West Africa. 

 Now we find that the brightness of these coronal rings 

 seems to depend upon proximity to the equator, and 

 is entirely independent of the prominences. That the true 

 spectrum of the corona will be eventually thus discerned 

 is unquestioned, and the sooner it is done the better. This 

 part of the attack this year has been greatly strengthened, 

 and not only have we here prismatic cameras of 6 and 9 

 inches aperture, but I have equipped Mr.Shackleton with a 

 powerful instrument for his observations in Novaya 

 Zemlya, W'hither he has gone in the expedition rendered 

 possible by the public spirit of Sir George Baden-Powell. 



The large scale prismatic camera was, as I have said, 

 introduced in 1893 — that is, only three years ago. The 

 results obtained in that year represent, therefore, only 



NO. 1400, VOL. 54 j 



the experimental stage ; at the critical moments of the 

 eclipse — that is, at the beginning and end of totality — 

 only snap-shots were taken. This time what is termed a 

 dropping-plate is introduced in the programme of the 

 9-inch. That will be exposed, while gradually falling, from 

 ten seconds before the end of totality to fifteen seconds 

 after, in the hopes of catching the so-called " flash " 

 which is supposed to represent the " reversing layer." 

 To my mind, the reversing layer is dead and buried 

 already, but may the fates be propitious on the gth, and 

 enable us to place a wreath on its tomb. 



So much then, briefly, for the prismatic cameras and 

 the pre-eminent importance of their use. I next come to 

 another point, to investigate which an important instru- 

 ment has already been set up. 



In organising the work for the eclipse of 1871, stress 

 was laid on the importance of obtaining a photograph of 

 all the light radiated earthwards during an eclipse, to 

 supplement the work of the slit spectroscopes which had 

 to do with the light radiated by special parts of the solar 

 surroundings. 



This work is a thousand times more important now 

 that the spectrum of the prominences is so clearly sepa- 

 rated from that of the corona by the prismatic cameras,, 

 because it enables us to make a flank attack, so to speak,, 

 on the corona spectrum. 



The integrating spectroscope to be used on the 9th 

 consists of a 4-inch Taylor lens of long focus as colli- 

 mator directed to the sun's place during totality in a way 

 I will state further on ; then come two prisms of 60" of 

 dense flint, and lastly a camera of 19 inches focus. The 

 light reflected from a dark cloud gives an exquisitely 

 sharp and well-dispersed solar spectrum in 40 seconds. 

 During totality a plate will be exposed for 60 seconds. 



Now in this instrument, simply pointed to the sun's^ 

 place, the light from the greatest area will give the 

 brightest lines. We may therefore expect the coronal 

 lines to be well visible ; and since the prismatic cameras- 

 are certain to give us a complete record of the chromo- 

 sphere and prominence spectrum, a simple subtraction 

 will bring us face to face with the spectrum of the upper 

 reaches of the solar atmosphere. 



I next come to another matter, on which it is necessary 

 to lay great stress. It is well known that Prof. Newcomb, 

 in 1878, introduced into eclipse work the use of a disc, 

 behind which the brighter lower layers of the sun's- 

 atmosphere, apparently surrounding the dark moon, were 

 hid during the totality. The object of this is, of course, 

 to shield the eye, and an additional precaution is to- 

 blindfold the observer till totality has well commenced. 



Armed in this way. Prof. Newcomb was enabled to see 

 long luminous extensions equalling in length several 

 diameters of the dark moon along the sun's equator. 

 Now since such long streamers had never been seea 

 before, it has been miagined that they indicate a special 

 form of corona visible at the period of minimum sun- 

 spot activity, for it was at very nearly this period that 

 the eclipse of 1878 occurred. 



But it may well be that the appearance may be due to- 

 the method employed, and that such an equatorial exten- 

 sion may be always there if only we can see it, and the 

 greater the solar activity the more difficult is it to see it 

 ordinarily, because this greater activity is always accom- 

 panied by a brighter lower corona. 



Prof. Newcomb, I believe, used a disc of such a size, 

 that the brighter lower corona some 3' above the dark 

 moon was covered. I hope to repeat this observation,, 

 and to extend it by using several discs, one or more of 

 which will cut off 5'. 



Finally, I have a few words to say on the various fea- 

 tures of the corona independently of the large exten- 

 sions, which can best be specially dealt with by the disc 

 observers. 



