September 7, 1905] 



NA TURE 



457 



THE TOTAL SOLAR ECLIPSE, AUGUST 30. 

 (i) The Solar Physics Observatory Expedition. 

 Palma, August 26. 



IN another four days the eclipse will be an event 

 of the past, and we shall be packing up the 

 great amount of material which we have been setting 



i. I. — Our camping ground as seen from 

 76 ft. prismatic reflector, and all the othe 

 discs in the right-hand corner. 



: south-east end. n the long tent 

 i beyond it. Notice 



up with so much care since .August 11. The greatest 

 keenness has been displayed in every party told off 

 for its particular duty, and I think that everyone will 

 be glad when the eventful day arrives. 



We have settled down to 

 routine work every day. Those 

 in charge of instruments go to 

 the camp at about 6.15, and 

 work at the adjustments and 

 small items so necessary for 

 successful photographs. .'\t nine 

 o'clock the whole band of volun- 

 teers, now about 150 in number, 

 arrives at the camp, and three 

 drills are then gone through in 

 fairly quick succession. The 

 organisation of the division of 

 labour at each instrument is now I 

 very satisfactory, and the various 

 movements that have to be 

 performed at stated times occur 

 fashion. 



As I have mentioned before, the whole work of the 

 camp is organised according to signals given by the 



have been previously calculated, give us 16 seconds 

 and 5 seconds respectively before the commencement 

 of totality. The object of employing these times is 

 not so much to assist the observers in the camp 

 generally, as to warn the workers with the prismatic 

 cameras, who begin making their e.xposures three 

 seconds before the commencement of totality. Both 

 Mr. Butler and myself utilise 

 these two signals to begin our 

 series of snap-shots for photo- 

 graphing the lower chromo- 

 sphere. 



Undoubtedly the three minutes 

 three seconds, the length of 

 totality at this station, is a long 

 time, and when the strong 

 voices of the timekeepers are 

 heard shouting out " 163 

 seconds," " 153 seconds more," 

 &c., one somehow feels that one 

 is not utilising to the fullest 

 extent the time available. 



With the prismatic camera, 

 of which I am in charge, 

 it is hoped to secure fourteen photographs. The three 

 large 6-inch prisms of 60°, and the object-glass of 

 the same diameter, form together a powerful weapon 

 of research. The programme of work is to make 



poles for the 



ght. All these instru 



clockwork 



Fig. ^. — The north end of the 

 room with the wine-tub for 

 the right, under the sm 



eclipse clock. There are, however, two further 

 signals given from the angles subtended by the cusp 

 at the centre of the dark room. These angles, which 

 NO. 1 87 I, VOL. 72] 



four snap-shots at about the commencement of totality 

 and five about the end. The remaining five plates, 

 will be exposed for intervals varying from 5 to 90 

 seconds, and it is hoped that the two long exposures, 

 on each side of mid-totality will add to our knowledge 

 of the wave-lengths of the coronal rings. This pris- 

 matic camera is designed to give resuUs suitable for 

 determining accurate wave-length of the chromo- 

 spheric and corona arcs ; the image of the sun is 

 therefore small, and the dispersion large. 



The prismatic reflector of 76 feet focal length, in 

 charge of Mr. Butler, provides a solar image of about 

 8 inches diameter, and, since the light is made to pass 

 through one prism twice, the dispersion is not 

 excessive. The large chromospheric arcs should, 

 however, provide us with much matter for thought. 



This latter instrument is practically ready for 

 the eclipse, and a few words may here be said as 

 regards the erection of it. The camera end itself 

 forms part of the dark room of the camp, and is to 

 the south of it. Just outside, but a little to the west 

 of the north and south line, is the siderostat, which 

 throws the solar rays on to the long-focus mirror 

 iituatcd to the south about 70 feet. This concave 

 reflector throws the image towards the north, into 

 the portion of the dark room in which is fitted a 

 screen. \n arrangeinent is adopted for inserting, 

 during some periods of totality, a prism in front of 

 the mirror. The light from the siderostat has thus 

 to pass twice through the prism, giving a very useful 



