DETERMINATION OF DEFLECTION OF LIGHT BY THE 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 with 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 coclostats 

 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 hope 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-,, 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 with 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. coxx. A. 2 T 



