298 SIR F. W. DYSON, PROF. A. S. EDDINGTON AND MU. C. DAVIDSON ON A 
in length, originally used by Father Corti?: 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- 
pendicidarity 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 screAved up to avoid any chance of subsequent movement. 
A feAV check plates of the field near Axcturus 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-diimnal 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 Avas very heavy rain, Avhich was Avelcome for its moistening 
effect on the ground, the dust hitherto having been troublesome to the clockAAmrk 
although every care had been taken to protect it. There Avas 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., Avas 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 Avere 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 Avas possible to reduce the temperature to about 75°, and by working 
