DETERMINATION OF DEFLECTION OF LIGHT BY T?IE SUN’S GRAVITATIONAL FIELD. 299 
only at night or before dawn development of the plates was fairly easy. Formalin 
was used in every case to harden the films, and thereby minimise the chance of distortion 
due to the softening of the films by the warm solutions. 
We had provided ourselves \vith two brands of plates, but it had become apparent 
from photographs taken and developed before the eclipse that one of these brands was 
unsuitable in the hot climate, and it was decided to use practically only one brand of 
plates. 
In taking the experimental photographs it was noticed that the clocks and coelostats 
were very sensitive to wind. We had reason to fear strong gusts about the time of 
totality, such as had occurred in other eclipses ; and as the conditions of our locality 
seemed to render them specially probable, protective wind screens were erected round 
the hut openings at every point where it was possible without interfering with the field 
of view. Happily dead calm prevailed at the critical time. Screens also protected all 
projecting parts of the telescope tubes from direct sunlight. 
The performance of the 16-inch coelostat was unsatisfactory in respect of driving. 
There was a clearly marked oscillation of the images on the screen in a period of about 
30 seconds. For this reason exposure time was shortened, so as to multiply the number 
of exposures in the hoj^e that some would be near the stationary points. 
13. The morning of the eclipse day was rather more cloudy than the average, and the 
proportion of cloud was estimated at -j-o at the time of first contact, when the sun was 
invisible ; it appeared a few seconds later showing a very small encroachment of the 
moon, and there were various short intervals of sunshine during the partial phase which 
enabled us to place the sun’s image at its assigned position on the ground glass, and to 
give a final adjustment to the rates of the driving clocks. As totality approached, the 
proportion of cloud diminished, and a large clear space reached the sun about one 
minute before second contact. Warnings were given 58s., 22s. and 12s. before second 
contact by observing the length of the disappearing crescent on the ground glass. When 
the crescent disappeared the word “ go ” was called and a metronome was started by 
Dr. Leocadio, who called out every tenth beat during totality, and the exposure times 
were recorded in terms of these beats. It beat 320 times in 310 seconds ; allowance 
has been made for this rate in the recorded times. The programme arranged was carried 
out successfully, 19 plates being exposed in the astrographic telescope with alternate 
exposures of 5 and 10 seconds, and eight in the 4-inch camera wdth a uniform exposure of 
28 seconds. The region round the sun was free from cloud, except for an interval of 
about a minute near the middle of totality when it was veiled by thin cloud, which 
prevented the photography of stars, though the inner corona remained visible to the 
eye and the plates exposed at this time show it and the large prominence excellently 
defined. The plates remained in their holders until development, which was carried 
out in convenient batches during the night hours of the following days, being completed 
by June 5. 
14. No observation of contact times was made, but it is known that these times were 
VOL. CCXX.-A. 2 T 
