146 



KNOWLEDGE. 



[July 2, 1900. 



to some 20 or 25 seconds. His object was two-fold ; to 

 get a rolling or grazing contact of the limbs of the sun 

 and moon whereby the " Flash " would be given out 

 all along the osculating surface ; and to get the 

 " Flash " not near the sun's equatorial regions, as do 

 the observers on the central line, but at the sun's pole. 

 Thereby it may be judged whether the constituents of 

 the sun's surroundings vary with their solar latitude. 



Mr. Evershed's Obseniiig Iliit nt Mazafram, sliowiiig the Ccelostat. 



rhoiotirajthci] hit E. Walter Maunder. 



In addition he used two large prisms in conjunction 

 with a large reflector. It is already a matter of history 

 liow that Mr. Evershed found himself, when the shadow 

 passed, about one hundred yards outside it, and not as 

 he had hoped two miles within. Though actually out> 

 side the total phase, he got some photographs of the 

 " Flash " of most exceptional beauty ; but probably not 

 one-fourth of the result which he would have got had 

 he had more accurate values for the joosition of the 

 shadow track. 



In another way his experience is of very great value, 

 though not by any means in the manner he intended 

 or desired. The farmers and sightseers in his near 

 neighbourhood had a vehement discussion as to whether 

 the eclipse was total or not. They divided themselves 

 into two parties, those who saw the sun completely 

 disappear, and those who described the coi-ona as creej)- 

 ing round to the moon up to a point where there was a 

 small remnant of sunlight. It got as far as this point 

 but no further, and straightway began to creep back 

 again and vanish. These latter also spoke of the shai-p 

 dividing line of light and shadow which sped across the 

 Mediterranean to their left hand. The division was, 

 or appeared to them to be, bordered by a bright line. 

 Investigation proved that both the parties were in the 

 right, for they had been separated by some five hundred 

 yards, the line of total phase passing between the inner 

 party and Mr. Evershed's tent. This unique obser- 

 vation affords a most accurate datum to correct the 

 computation of solar eclipses in the future. It is a 

 pity, however, that such an observation should have 

 been at the expense of Mr. Evershed's special re- 

 searches. 



Besides Mr. Evershed at Mazafram, Mr. Newall, Prof. 

 Turner and Mr. Wesley at Bou Zarea, and a strong 

 party of Swiss and Italian astronomers at Menerville, 

 more than twenty members of the British Astronomical 

 Association took up their quarters in the city of 



Algiers itself, and observed with us from the roof of 

 the Hotel dc la Regence. 



Our own particular work was photographic. It may be 

 remembered that our programme in India was to take 

 in duplieate a series of graduated exposures varying in 

 equivalent efficiency from 1 to 1800. Our hope had been 

 that the longer of these exposures might secure the faint 

 coronal extensions, but our purpose was in any case to 

 learn more than had yet been done as to the real 

 efficiency of different exposures in coronal photography. 

 It was the first time, it was the only time, that such 

 an attempt had been made except within quite narrow 

 limits. 



It is well known that we were doubly fortunate. 

 Our series included six different exposures equivalent 

 to 1, 4i, 20, 90, 400, 1800, each exposure being given 

 in duplicate so that twelve plates were exposed in all. 

 From the twelve plates we secured four photographs 

 successful as such, but each plate had its full value as 

 a lesson in exposures, and three recorded the long rays. 



Seeing that our longest exposures were the most 

 successful in bringing up the long rays, the question 

 before us this time was, Had we reached the limit in 

 India of successful exposure, or had we not? If not, 

 how far could we extend it? 



Here we met a serious difficulty, for totality at 

 Algiers would last barely 62 seconds, and we could not 



On till- Roof of tlie Hotel de la Eegence, Algiers. Rev. C. D. P. 

 Davies and Telephotograpbie Camera. 



Pliotoji-ajilirrl !■!/ Miss Emth Maunder. 



expose for the entire time. It was moreover our prin- 

 ciple to trust nothing to a single plate ; we resolved 

 to adhere strictly to our Indian precedent in this respect 

 and make every exposure in duplicate. 



We got over the difficulty in this way. We pur- 

 chased a second Dallraeyer stigmatic lens of the same 

 aperture and focal length as that used in 1898, 

 and we exposed a plate with each for 48 seconds 

 instead of 20 seconds in India. This was to increase 

 the exposure in the ratio of 12 to 5. We were 

 anxious, however, not to confine ourselves to a pair of 

 exposures of one length only, and consequently pur- 

 chased a pair of R.R. lenses of focal length double that 

 of the stigmatic, and which we used at the same aper- 

 ture as we had done the former, namely, \\ inches. 

 The effective exposure therefore for these lenses — which 

 we also exposed for 48 seconds — was but one-quarter 

 that of the stigmatic or three-fifths the longest exposure 



