MOUNT WILSON SOLAR OBSERVATORY. 281 



Koch registering micro-photometer, with bed 12 feet long, has been 

 completed, and much other work accomphshed. This includes many- 

 improvements in the coelostat and second mirror support of the 60-foot 

 tower telescope, the construction of running gear, driving mechanism, 

 and other parts for three domes, a large figuring attachment for the 

 100-inch grinding machine, completion and erection of the 15-foot 

 concave-grating spectrograph, and much work on the 10-inch portrait- 

 lens mounting, 4 by 5 inch comparator, two 8-inch measuring machines, 

 vacuum-spark apparatus for the large electromagnet, machine for 

 grinding rails of dome for 100-inch telescope, addition to 8 by 10 plate- 

 carrier, addition to Cassegrain spectrograph, experimental work on 

 liquid prism, repairs and alterations of instruments, repairs on power 

 plant and auto truck, repairing and scraping of ways and bearings of 

 56-inch lathe, etc. 



A Cincinnati No. 1| motor-driven cutter and tool-grinder, a large 

 motor-driven power-saw equipped to cut 24-inch sheets and 18-inch 

 rounds, and a 2 horse-power motor Aerial grinder were added to the 

 instrument-shop equipment. 



THE ONE-HUNDRED-INCH TELESCOPE. 



Optical work on the 100-inch mirror has been continued by Mr. 

 Kitchey during the year, and an almost perfectly spherical figure has 

 now been attained. The edge support referred to in the last annual 

 report has been found to work excellently, and no distortion of the 

 mirror due to this source has been observed in recent tests. A very 

 slight amount of astigmatism apparently still remains to be corrected, 

 but the zonal errors have been almost completely eliminated. It is 

 thought that if a spherical figure entirely free from zones can once 

 be secured it will be possible in the course of parabolization to keep the 

 mirror essentially free from such errors. 



As the work on the spherical figure of the mirror has progressed it 

 has been found necessary to adopt certain additional precautions to 

 protect it from temperature variations. Thus, in spite of attempts to 

 produce a thorough mixture of the air in the optical room and in the 

 testing hall by means of fans, there is sufficient stratification to affect 

 the tests seriously, as well as sufficient temperature variation between 

 the top and the bottom of the mirror, when in a vertical position, to 

 introduce a small amount of distortion. To improve the first of these 

 conditions a number of large tubes made of paper over a wooden 

 framework have been distributed throughout the optical room, through 

 which a circulation of air is obtained by means of fans. Considerable 

 improvement is noticed in the tests when this arrangement is used. 

 To protect the edge of the mirror a thick band of woolen wadding 

 inclosed in canvas was placed between the edge band and the glass. 

 A similar cushion beneath the glass affords excellent protection to the 



