372 REPORTS OF INVESTIGATIONS AND PROJECTS. 



By the use of a knife-edge, employed as in the Foucault method of testing 



the figure of mirrors, Professor Ritchey finds no difficulty in determining the 



focal plane within 0.001 inch. As this is only —^— of the focal length, 

 ^ -' 300000 ° 



such precision is most satisfactory. If the focal length changes during the 

 exposure, the plate-holder is removed from time to time and the knife-edge 

 employed to refocus. The evidence goes to show that the changes in focal 

 length are due mainly to the expansion and contraction of the tube, and not 

 to the 60-inch mirror. Hence the scale of the image does not change, if the 

 plate is kept at a fixed distance from the mirror. 



The nebulse thus far photographed in the principal focus include the 

 Great Nebula in Orion, the Great Nebula in Andromeda, the Pleiades, the 

 Ring Nebula in Lyra, the Dumb-bell Nebula A''. G. C. 6960, N. G. C. 6992, 

 M. 33 TrianguH, and M. i Tauri. 



As yet there has been little opportunity for photography with the Casse- 

 grainian combination of 100 feet focal length, but the results obtained are 

 most encouraging. From an optical standpoint, this arrangement has been 

 found perfectly satisfactory. The star images are so sharp and symmetrical 

 as to leave no doubt regarding the excellence of the figure of the hyper- 

 boloidal mirror and that of the plane mirror at the intersection of the polar 

 and declination axes. 



Stellar Spectroscopy. 



It has long been my purpose to photograph the spectra of the brighter 

 stars on the scale of the negatives used in making Rowland's map of the 

 solar spectrum. The solution of this problem would render possible the 

 determination of the pressure existing in stellar atmospheres, and many 

 other investigations which are beyond the reach of existing stellar spectro- 

 graphs. Experiments with this end in view were accordingly begun at the 

 Yerkes Observatory in 1901. These were continued by Mr. Adams and 

 myself on Mount Wilson in 1905, with the aid of the Snow telescope. The 

 results of this work have already been published (Contributions from the 

 Mount Wilson Solar Observatory, Nos. 8 and 12), In the design of the 

 mounting of the 60-inch reflector, provision was made, in one of the Casse- 

 grainian combinations, to send the light down through the polar axis, after 

 reflection at a plane mirror mounted at the intersection of the polar and 

 declination axes. The star image is thus formed at a point some distance 

 below the lower extremity of the polar axis ; the equivalent focal length of 

 the mirror system is 150 feet. The spectrograph employed to photograph 

 this image may be mounted within a concrete house, on a pier whose upper 

 surface is parallel to the polar axis. A different arrangement permits the 

 use of a vertical spectrograph, in which the prism or grating is mounted 

 in a constant temperature chamber about 14 feet below the level of the 



