MOUNT WILSON 80LAU OBSIORVATORY. 237 



100-inch reflector mounting. As mentioned above, the focal-plane 

 spectrograph for this instrument is now well under way, and patterns 

 are now being made for the driving-clock castings. 



PASADENA OFFICE-BUILDING. 



The Observatory has enjoyed few advantages greater than those 

 which have resulted from the occupation of the new office-building in 

 Pasadena, which has been in use since January. The very small and 

 crowded offices formerly available in the shop-buildings were often in- 

 tensely hot during the summer months, to the serious detriment of the 

 comfort and the efficiency of the staff. In the new building, on account 

 of its massive construction, the temperature conditions are greatly 

 superior, though the past summer has been exceptionally hot. 



One of the most useful and attractive features of the new office- 

 building is the library, where for the first time the books of the 

 Observatory have been made readily accessible. An excellent copy 

 in oils of Collier's well-known portrait of Sir William Huggins, made 

 when he was President of the Royal Society, was offered by a friend 

 of the Observatory for the decoration of this room. The library has 

 grown steadily and now includes many sets of journals and publica- 

 tions of observatories, laboratories, and learned societies. Removed 

 as we are from the great libraries of the eastern cities, it is important 

 to extend this collection in several directions, and to complete imper- 

 fect sets, which will be done as rapidly as possible. 



THE ONE-HUNDRED-INCH TELESCOPE. 



In the last annual report the peculiar changes of figure observed 

 when testing the 100-inch mirror were described. When the glass was 

 hung on edge for testing, the figure was found to alter when the disk 

 was rotated in its own plane. The observed effect was attributed 

 by Mr. Ritchey to different degrees of compression of the disk along 

 different diameters, under the pressure of its own weight. 



To test the compression hypothesis a weight of 5 tons was sup- 

 ported on the upper edge of the glass, at the position angle corre- 

 sponding to the "weak" diameter. When the point of support was 

 brought into the neutral plane of the glass, found by experiment to 

 be 5f inches from the rear surface, the figure was seen to be pre- 

 cisely the same as that observed when no pressure was applied. On 

 either side of this plane the pressure introduced a component at right 

 angles to the face of the disk, and thus caused a distortion of the 

 glass. The solution of the whole difficulty was thus immediately 

 suggested. It was found that the edge-band support, though made 

 with unusual care, had not been properly applied. As soon as this 

 was discarded and the glass allowed to rest on a shore-block of wood, 

 with the point of support lying in the neutral plane, the figure was 

 not changed in the least by rotation of the disk. Incidentally it 



