156 REPORTS OF INVESTIGATIONS AND PROJECTS. 



The instrument and optical shops were closely occupied with the 60-inch 

 reflector during the entire year. The optical work included the grinding and 

 figuring of the two plane and two hyperboloidal mirrors, which were tested 

 in combination with the 60-inch mirror. In the instrument shop the mount- 

 ing was completed and erected, so that the operation of all parts of the mech- 

 anism could be thoroughly tested before they were sent to the mountain. In 

 the tests the various quick and slow motions, effected by electric motors, 

 worked perfectly, and the mounting showed no evidences of flexure. 



The experience of the previous year had shown the necessity of widening 

 some sections of the Mount Wilson road, in order to permit the large and 

 heavy parts of the mounting to be carried safely on the avitomobile truck. 

 This work was done in the spring, immediately after the close of the rainy 

 season. At the same time the erection of the steel building and dome was re- 

 sumed. During June and July the mounting was taken to the summit with- 

 out difficulty on the truck, though four strong mules were needed to assist the 

 engine in hauling the heaviest loads, weighing 5 tons, over the steep grades. 

 The most troublesome load was the large steel telescope-tube, 6.5 feet in di- 

 ameter and 18 feet long, which was taken up as a single piece. All parts of 

 the telescope, including the 60-inch mirror, reached the summit without the 

 slightest injury. Work on the dome and building was completed early in 

 September, and the erection of the telescope is so far advanced that it should 

 be ready for use this autumn. 



A spectroheliograph of 30 feet focal length, designed for use with the tower 

 telescope, has just been completed in the instrument shop. The great linear 

 dispersion of this instrument, and the fact that it will permit three photo- 

 graphs of the same region of the sun to be taken simultaneously with the 

 light of three different lines, should prove advantageous in certain new fields 

 of solar research. The dispersing member of this spectroheliograph is a large 

 fluid prism, with circular faces 12 inches in diameter, which will be twice 

 traversed by the light (Littrow arrangement). 



After repeated trials the plate-glass works of St. Gobain, France, has finally 

 succeeded in casting a suitable glass disk for the lOO-inch mirror of the 

 Hooker telescope. This is now on the way to Pasadena. As the large grind- 

 ing machine, constructed for this mirror, is nearly completed, the work of 

 grinding the disk should soon be under way. 



The new spectroscopic laboratory, built and equipped by the construction 

 division, and mentioned elsewhere in this report, is described in Contributions 

 from the Mount Wilson Solar Observatory, No. 27. 



