360 Rev. A. L Cortie [Feb. 26, 



of l-l-iiich focus, with which he successfully photographed on " Hydro '' 

 plates the moon projected on the corona, 30 seconds, and 1 minute 

 after totality. The function of the 3-inch Cooke telescope mounted 

 on top of the long camera was to enable Father O'Connor to give 

 signals U) minutes, 5 minutes, and 10 seconds before totality, by 

 observation on the graduated glass screen of the angles subtended by 

 the cusps of the advancing moon at the centre of the sun's projected 

 image. An ingenious eclipse-clock constructed by Mr. (libbs, a large 

 hand completing the circuit of a two-foot dial once in 129 seconds, 

 was also set up. It was illuminated by an electric lamp. 



The 8-inch coelostat was employed to supply light to a 5 -inch 

 Alvan Clarke lens, focal length 7 feet, which projected an image of 

 the sun on to the slit plate of the spectrograph. This spectrograph 

 was designed in conjunction with Professor Fowler, and was constructed 

 under his supervision at the Imperial College of Science, South 

 Kensington. It was probably the most powerful slit spectrograph so 

 far used in eclipse expeditions, and gave a very brilliant spectrum 

 covering -4 inches lineally between X 6700 and X 4800. It was of the 

 Littrow type, and the constants of the instrument were as follow : — 



This instrument was worked by Father Cortie and Mr. Gibbs. 



A novel feature in eclipse operations was the photographing of a 

 comparison spectrum given by an iron-arc on the same plate as the 

 spectrum of the corona. A current of 12 amperes at a pressure of 

 110 volts was employed. The image of the arc was formed on the 

 slit by means of a single lens, 4 • 9 inches aperture, and a diagonal 

 prism. The diagonal was mounted in front of the lens so that it 

 could be pushed forwards and backwards in a slotted groove. To fill 

 the O.G. with light from the arc, the lens was adjusted so that the 

 solid angle of the beam of light from the arc was equal to that sub- 

 tended by the O.G. from the slit. A zinc shield with a horizontal 

 slot ^^ inch wide was employed to cover the slit plate, during the 

 exposure of the corona, when it was desired to expose for the spectrum 

 of the arc. The slot lay across the dark image of the moon. The 

 coronal image was placed on the slit plate so that the slit was almost 

 tangential to the sun's limb on the E. side. The exposure on the 

 spectrum of the corona had been in progress about 40 seconds, when 

 the slotted shield was placed in position, the diagonal was pushed 

 forward, and the arc struck. After an exposure of 4 seconds on the 



