MOUNT WILSON SOLAR OBSERVATORY. 20O, 



mospheric conditions in so large an instrument, might possibly be feared. 

 But, as already stated, experiments in the dome of the 6o-inch show that 

 there is no cause for apprehension on this score, and improvements in the 

 new design will certainly eliminate any outstanding difficulties. Further- 

 more, in the new plan all the mirrors (and the tube, if necessary) are to be 

 kept at a constant temperature throughout the day and night, thus preventing 

 such changes of figure as would certainly have occurred if the mirror had 

 been exposed to the ordinary fluctuations in temperature of the night air. 



All of these considerations will be of no moment, however, unless a suit- 

 able ioo-inch mirror can be made. In the last annual report it was stated 

 that the French Plate Glass Company had failed in all of its attempts to cast 

 a disk free from internal flaws, and that accordingly the disk sent to us some 

 years ago was being ground to a spherical figure, in the hope that optical 

 tests might prove it to be suitable for use in the telescope. Mr. Ritchey's 

 illness unfortunately prevented him from resuming work until January i, 

 but the fine grinding was continued without other interruption than that in- 

 volved in the installation of a ventilating and heating system, with water- 

 spray air-filter and thermostat control, in the ioo-inch optical shop. For the 

 work of polishing, it became necessary to make a polishing tool ioo inches in 

 diameter to supplement the smaller tools. 



As soon as an approximately spherical surface had been obtained, a regu- 

 lar series of optical tests was begun, with the mirror hanging on edge in a 

 thin, heavily cushioned steel band. Effects of astigmatism were observed, 

 which seemed to vary in magnitude when different diameters of the disk 

 were vertical. The observed effects are so complicated that no satisfactory 

 explanation can yet be given. Among them may be mentioned astigmatism 

 of the surface, apparently not permanent in amount ; flexure, different along 

 different diameters, when the glass is turned on edge for testing, and change 

 of figure caused by the daily variation in temperature of the optical shop 

 (about 2°C). 



On August 20 a 36-inch plane mirror was suspended at 45 ° above the ioo- 

 inch mirror, permitting the latter to be tested for astigmatism while lying 

 horizontal. A long series of determinations of radius of curvature, made 

 with the greatest precautions on two days, showed the surface to be slightly 

 astigmatic. The maximum difference of radius of curvature, for two diame- 

 ters (x and y) of the glass at right angles to each other, was 0.06 inch 

 (x > y). When the glass was hung on edge with the x diameter vertical, 

 the radius of curvature of the x diameter was 0.025 mcn greater than that 

 of the y diameter. With the y diameter vertical, the radius of curvature of 

 the x diameter was 0.137 inch greater than that of the y diameter. Since the 

 date of the last tests about two-thirds of the astigmatism has been removed. 



The changes in figure due to variations in the temperature of the optical 

 room could probably be eliminated by the use of a suitable constant-tem- 

 perature cell. However, if clear evidence of irregular flexure persists after 



