20.t 



KNOWLEDGE 



[Septeubeb 1, 1896. 



of tlie appearance of the light during totality as to enable 

 those who have not seen an eclipse to realize it. One 

 observer spealcs of " sea, sky, and hills all becoming an 

 intense livid blue"; others preferred to call it the "deepest 

 indigo purple" they had ever witnessed; others again 

 differentiated between the tone of cloud and laud, and 

 spoke of the former as being of a " cold dark-grey black," 

 whilst the hills retained in their blackness some tinge of 

 blue or purple. lUit there was a general opinion that 

 the colour of the amber and ruddy rifts were not only 

 like " sunset colours," but had identically the same origin. 

 Mr. Green nrges that the golden light so conspicuous at 

 sunset is always present at its proper low altitude in the 

 sky ; it is only that it becomes more conspicuous when 

 the daylight fades as the sun sinks below the horizon. 

 On this hypothesis there is no need for wonder that the 

 amber and ruddy tints seen in the low-lying rifts appeared 

 so vivid. This would naturally follow as an effect of 

 contrast with the dark masses of cloud and rock above and 

 below them. 



And now for the lessons of the eclipse, for though we 

 were not fortunate enough to see it, our experiences have 

 their lessons. 



First of all, the fact that a magnificent view was afforded 

 at Bodo, where the small altitude of the sun rendered 

 success so unlikely, whilst at Vadso, and at Bugonu'S, 

 where the chances seemed reasonably good, the eclipse 

 was hidden by clouds, reinforces and drives home the 

 lesson taught by a dozen previous eclipses, that no 

 accessible station whatsoever must be left unoccupied ; 

 and ;hat those who are sufficiently self-denying as 

 to adopt a location apparently hopeless, may, in spite 

 of meteorological reports, carry off the prize even before 

 those who have stationed themselves where all seemed 

 promising. 



Another lesson, not less important, is that of the value 

 and necessity of drill. The work accomplished by Prof. 

 Lockyer and his assistants at Syd Yaranger was most 

 remarkable, and calls for very full recognition. The 

 organization of practically an entire ship's company as an 

 observing staff, and their training into a state of thorough 

 preparedness, was a most remarkable achievement. 



If Mr. Lockyer's achievement was surpassed, then I 

 think the British Astronomical Association may lay claim 

 to that merit. In many ways the task before the officers 

 of the Association was a far heavier one than that before 

 Prof. Lockyer. The number of observers to be brought 

 into line was considerably larger, and these were not naval 

 officers well accustomed to strict discipline and exact 

 obedience, but independent ladies and gentlemen out on a 

 holiday excursion. Nevertheless the task was accomplished. 

 The observers were organized, trained, and exercised, and 

 on the morning of the eclipse each was in his or her 

 appointed place, knowing what to do, and confident, from 

 the rehearsals that had taken place, as to their ability to 

 accomplish it. That this was possible was due, lirst, to the 

 unsparing earnestness of those undertaking the work of 

 organization — Br. Downing, Messrs. Crommelin, Evershed, 

 Green, and Lunt "Wesley, and the Rev. J. Cairns Mitchell 

 — and, next, and not less, to the most cheerful and ready 

 co-operation and help, not only of those who had come out 

 expressly as observers in connection with the British 

 Astronomical Association, but also those who had come 

 merely to make a holiday, and to see an unwonted 

 spectacle. 



Lastly, I think that it is clear that the equatorial in its 

 ordinary form will be less and less the eclipse instrument 

 of the future. In some cases the most convenient device 

 will be that adopted by Prof. Schaeberle, in 189.3, and by 



Dr. Copeland on the present occasion, of a fixed telescope 

 and a travelling plate, the motion of the plate being 

 regulated to compensate for the motion of the sun. In 

 many respects, a better way of getting over thedilliculties 

 of a fixed telescope is by the use of an auxiliary mirror, 

 mounted in one of several ways. The ordinary heliostat 

 has the drawback that it is not suitable for any but very 

 short exposures, on account of the apparent revolution of 

 the imago. The double heliostat overcomes this ditiiculty, 

 but at the cost of a second deflection. The polar heliostat 

 requires the telescope to be parallel to the polar axis, often 

 a very inconvenient arrangement. On the whole, the 

 Ctt'lostatic method, employed for the first time in this 

 eclipse, appears to offer great possibilities, and will 

 probably obtain greater favour as time goes on. 



For those, however, who have only small instruments 

 at their disposal, and especially cameras in which the focal 

 length is small and the aperture relatively large, there can 

 be no doubt that the best plan, in default of the assistance 

 of a cu'lostat or its equivalent, will be to fix the instru- 

 ment rigidly pointing to the sun, to dismiss all idea of 

 following, and to limit the exposures, so that tho blurring 

 due to the apparent motion of the sun v^ould not be 

 appreciable in the time. 



The above remarks apply, of course, only to photographs 

 of the corona itself. The inner portions of the corona are 

 so bright, and the best modern plates so sensitive, that an 

 exposure practically instantaneous is sufficient to obtain a 

 good record. The experience of former eclipses shows 

 that the tendency has been distinctly to over-expose — even 

 with the less rapid plates formerly available— and, indeed, 

 save under the most exceptional conditions of sky, a limit 

 is soon reached in which further exposure, instead of 

 bringing up more coronal features, only brings up the 

 general sky illumination. It has become, therefore, clear 

 that short exposures must be the rule, and, if short, there 

 is the natural desire to obtain as large a number of these 

 as possible. The crucial question then becomes, how to 

 obtain the greatest possible number of exposures, without, 

 in the process of changing plates, setting up such tremors 

 as will destroy all definition '? To effect this it is of first 

 necessity that the telescope itself be as stable as possible, 

 and this can be far better secured where it is immovably 

 fixed than where it is equatorially mounted and driven by 

 clockwork. 



As to the methods of changing plates, there seems little 

 advantage in one over another. Perhaps the method 

 which promises best is that of a separate dark slide for 

 each plate, the dark slide being made to rest on the end 

 of the camera, not to fit into a tight groove — to be held in 

 its place by an easily moved spring, and the shutter of the 

 slide to open door fashion. The changes in this case will 

 probably be made as quickly, if not more so, than by any 

 arrangement of changing boxes, revolving drums, or long 

 continuous slides ; it will be much less weighty, less liable 

 to jar, and free from all possibility of sticking at a critical 

 moment. 



For spectroscopic work, the conditions are quite different, 

 and vary with the different departments of work to be 

 attempted. Here the equatorial may still hold its ground, 

 though the balance of convenience will be greatly on the 

 side of the coelostat. 



Lastly, the one great lesson which the disappointment 

 of Vadso seemed to enforce upon all those who suffered 

 from it, was to leave no stone unturned to secure that 

 they should take part in the observation of the next solar 

 eclipse, that visible in India in January, 1898, whence, so 

 far as mortal can foresee, there will be little or no fear of 

 the untoward weather that battled our efforts in Finmark. 



