MOUNT WILSON SOLAR OBSERVATORY. 177 



inches (11.4 cm.), thus yielding a valuable additional prism for use in the 

 focal plane stellar spectrograph. 



A rheostat has been provided to vary the speed of the spectroheliograph 

 motor, thus giving a more perfect control of the exposure-time than the 

 system of change-gears formerly permitted. 



The 150-F00T Tower Telescope. 



The year has marked the completion of this instrument, after long delays 

 occasioned by the difficulty in obtaining a suitable object-glass of 150 feet 

 focal length. As mentioned in the last report, the 12-inch triple objective at 

 first supplied by the makers was defective and had to be replaced by a visual 

 objective of the ordinary two-lens type. This gave poor definition, and new 

 glass disks were obtained for a third attempt, which proved to be very suc- 

 cessful. The third objective was delivered by Brashear in May, and was 

 immediately mounted in the telescope, where it has been in regular use for a 

 variety of investigations. 



The 75-foot combined spectrograph and spectroheliograph was completed 

 in December, and the 12-inch objective, of 60 feet focal length, which be- 

 longs to the old tower telescope, was then transferred to the new tower, in 

 order that observational work might begin before the completion of the 150- 

 foot focus objective. The preliminary tests were most satisfactory in all 

 respects save one — the focal length varied at different hours of the day, and 

 the solar image gave evidences of astigmatism near noon. Experience with 

 the 60-foot tower having shown that the rise in temperature of the mirrors 

 from night to day would probably produce such effects, water-jackets sur- 

 rounding their edges were provided, but during the winter these were not 

 needed. In July the higher temperature and the use of the 150-foot focus 

 objective rendered the changes far more conspicuous. The slight astigma- 

 tism previously found gradually grew more marked, and finally it became 

 evident that the image was not merely astigmatic, but multiple. This was 

 accompanied by increased focal change, amounting in some cases to 5 feet 

 (1.5 m.) in a few hours. In the hope of preventing such changes, a steady 

 circulation of water, kept at a nearly constant temperature, was maintained 

 in the water-jackets covering the edges and backs of the mirrors. As soon 

 as equilibrium was established the astigmatism completely disappeared and 

 the change of focal length was reduced to a small fraction of its former 

 amount. Long exposures with the spectroheliograph may now 7 be given 

 without interference from this cause. 



On account of the great height of the new tower (176 feet to top of 

 dome) doubts have been expressed by various writers as to its stability. The 

 w r ork of the last year proves these fears to be groundless. So delicate an op- 

 eration as the photography of the "flash" spectrum has been carried out 

 when the wind was blowing 20 miles an hour, and there has never yet been 

 an occasion when it was necessary to stop work because of trembling of the 

 image. In another respect the tower has also met our hopes. The defini- 



