298 SIR F. W. DYSON, PROF. A. S. EDDINGTON AND MR. C. DAVIDSON ON A 



in length, originally used by Father COBTIE at Hernosand in 1914, together with the 

 10 X 8-inch plate carriers. Study of the star-diagram showed that seven stars could 

 be photographed by turning the plate through 45 degrees. The tube was therefore 

 placed on its angle, large wooden V-supports being prepared to fit the tube ; these 

 rested on strong wooden trestles. 



The focussing was at first done visually on Arcturus, using an eyepiece fitted with 

 a cobalt glass (after the plate supports and object-glass had been adjusted for per- 

 pendicularity to the axis). A series of exposures was then made, the focus being varied 

 slightly so as to cover a sufficient range. Examination of these photographs showed 

 at once that there was serious astigmatism due to the figure of the mirror of the 16-inch 

 coelostat. By inserting an 8-inch stop this was reduced to a large extent, and this 

 stop was henceforth used throughout ; but the defect was of such a character that it 

 was clear that it would be necessary to stay at Sobral and obtain comparison plates 

 of the eclipse field in July when the sun had moved away. 



The focus of the 4-inch was determined in a similar manner. The images, though 

 superior to those of the astrographic, were not quite perfect, and here again com- 

 parison plates in July were necessary. Once the focus had been decided on, the breech 

 end was securely screwed up to avoid any chance of subsequent movement. 



A few check plates of the field near Arcturus were taken, but have not been used. 



11. The following is a summary of the meteorological conditions during our stay. 

 The barometer record was interesting in that it showed very little change from day to 

 day, in spite of changes in the type of weather ; there was, however, a very well marked 

 semi-diurnal variation, with range of about 0-15 inch. The temperature range was 

 fairly uniform, from a maximum of about 97 F. towards 3 p.m. to a minimum of about 

 75 F. at 5 a.m. The relative humidity (as shown by a hygrograph belonging to the 

 Brazilian Commission) followed the temperature closely, varying from 30 per cent, in 

 the afternoon to 90 per cent, in the early morning. 



May is normally the last month of the rainy season at Sobral, but this year the rainfall 

 was very scanty ; there were a few afternoon showers, each ushered in by a violent gust 

 of wind ; and on May 25 there was very heavy rain, which was welcome for its moistening 

 effect on the ground, the dust hitherto having been troublesome to the clockwork 

 although every care had been taken to protect it. There was a fair amount of cloud 

 in the mornings, but the afternoons and nights were clear in the majority of cases. 

 Mt. Meruoca, 2,700 feet high, about 6 miles to the N.W., was a collector of cloud, its 

 summit being frequently veiled in mist. In spite of its cooler climate, the summit 

 would thus not have been a suitable eclipse station, and, in fact, nothing of the total 

 phase of the eclipse was seen from it. 



12. Although water was generally scarce, we were very fortunately situated as we 

 enjoyed an unlimited supply of good water laid on at the house. This was of great 

 benefit in the photographic operations. Ice was unobtainable, but by the use of earthen- 

 ware water-coolers it was possible to reduce the temperature to about 75, and by working 



