186 



STELLAR PHOTOGRAPHY. 



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ary connection gave the signals uninterruptedly. Finally, the clock was arranged 

 as a make-circuit and the governor required a break-circuit. This difficulty was 

 remedied by throwing the clock out of the circuit and using it as a shunt to the 

 battery. The circuit was thus closed both through the clock and the governor. 

 The clock resistance was so small that but little of the current ordinarily passed 

 through the governor. Every two seconds the clock circuit was broken, and the 



passed through the governor. An incidental advantage of 



© 



ment is, that, as the circuit is never entirely broken, no spark occurs within the 

 clock. 



The frame carrying the polar axis rested on a bed-plate with adjusting screws 

 so that it could be raised or lowered at either corner or moved in azimuth. The 

 whole rested on a large stone imbedded in the ground and covered by a transit shed, 

 which had been constructed for observing the Transit of Mercury in 1878. All the 

 photographs were taken near the meridian, where the atmospheric absorption and 

 refraction are least and vary most slowly. The stone did not prove to be sufficiently 

 immovable when subjected to frosts and thaws, and consequently a circular level 

 was attached to the frame of the instrument. The polar axis could thus always 

 be brought back to the same altitude, whatever the motion of the stone. 



The transit shed covering the instrument originally had a slit only one foot in 

 width. This was widened to two feet and closed by two light shutters covered with 

 canvas. It was expected that this would permit stars on the equator to be reached 

 when one hour east or west of the meridian. When the instrument was mounted, 

 the axis was found to be lower than had been anticipated, and the edges of the slit 

 began to interfere when the telescope was no more than twenty minutes from the 

 meridian. This difficulty was remedied in March, 1886, when the instrument was 

 remounted. It was then placed on the pier used during the Transit of Mercury, 

 which proved a much steadier support. It also permitted exposures of over an hour 

 on the equator, and longer exposures near the pole, without interfering with the 

 edges of the shutter. 



All the photographs were taken on bromo-gelatiue dry plates, having the dimen- 

 sion of eight inches by ten. They accordingly covered a region of 12° in declination 

 and 10° in right ascension. On the equator this represents forty minutes of time. 

 Most of the plates were made by Messrs. Allen and Rowell, of Boston, and were of 



the form 



the " Extra Quick." Cramer's developer was used 



pyrogallic acid and carbonate of soda. 



The photographic work may be divided into three classes. First, the telescope 



