Solar Eclipse of May 28, 1900. 375 



These relatively long exposures were designed to secure density in 

 the ultra violet, at the risk of over-exposing in the region near G. 



Having ascertained by rehearsing that the time required for exposing 

 all the plates would be about 90 seconds, I arranged that the first 

 exposure should be timed at 45 seconds before the computed time of 

 mid-eclipse The succeeding exposures were to follow each other at 

 the shortest intervals, turning the handles deliberately, and allowing 

 ample time for shake to subside before each exposure. 



I was to use my discretion to some extent in making the long 

 exposure at mid-eclipse, but otherwise I intended to be guided solely by 

 the chronometer. 



The exposures were to be made by myself, standing at the north 

 door of the hut and facing the large spectrograph. I used my left 

 hand to work the exposing shutter, and my right to rack the plates 

 forward in the slide. 



My brother, sitting on his bed in the hut, was to move the plates of 

 the 2-inch spectrograph, turning the handle half a revolution after 

 each exposure. He was also to expose the quartz spectrograph at a 

 signal from me. 



The Coelostat. 



A 12-inch ccelostat was used to supply light to the three spectro- 

 graphs in the hut. It was placed about 6 feet from the north-east 

 corner of the hut, and was arranged to reflect the sun in a meridional 

 plane ; the angle between the incident and reflected beam being in 

 this case a minimum, viz., declination of sun x 2. The reflected 

 "beam was in a direction W. 30 S., and was directed upward at an 

 angle of 3| with the horizontal. 



The instrument was mounted on a steel plate fixed on the top of a 

 masonry pier, and about 3 feet from the ground. The plate had a 

 straight channel cut in it just wide enough to take the ends of two of 

 the four levelling screws, the other two resting on the planed surface 

 of the plate. 



With the plate placed approximately level and the channel approxi- 

 mately north and south, the whole instrument could be shifted bodily 

 north or south without disturbing the adjustments of the axis in 

 altitude and azimuth. In this way the adjustment of the beam of light 

 with respect to the apertures of the three spectrographs was very 

 easily managed, and the coelostat could be shifted about to suit the 

 varying declination of the sun on occasions, previous to the eclipse, 

 when it was desired to observe the spectrum and adjust the spectro- 

 graphs. 



The crelostat was provided with slow motion independent of the 

 driving gear, and this was controlled from inside the hut by means of a 

 rod, 8 feet long, which my brother laboriously cut from a 3-inch plank. 



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