ASTRONOMY — HALE. 59 



the work of construction is carried on under the supervision of 

 Prof. G. W. Ritchey, astronomer and superintendent of instrument 

 construction. 



Prof. Winslow Upton, Director of the L,add Observatory of Brown 

 University, and Prof. L,. H. Gilmore, of Throop Polytechnic Insti- 

 tute, were engaged in special work at the Solar Observatory during 

 the summer of 1905. 



During 1905 the following expeditions have conducted observations 

 on Mount Wilson in cooperation with the Solar Observatory: 



Hooker Expedition: Edward E. Barnard, astronomer of the 

 Yerkes Observatory, in charge. 



Smithsonian Expedition: Charles G. Abbot, aid acting in charge of 

 the Smithsonian Astrophysical Observatory, in charge ; Leonard R. 

 Ingersoll, University of Wisconsin, assistant. 



INVESTIGATIONS IN PROGRESS. 



Although the program of observations must naturally be a re- 

 stricted one until the completion of the instrumental equipment, it 

 has nevertheless been possible to commence systematic work in four 

 departments : (1) daily photography of the sun with the photohelio- 

 graph ; (2) daily photography of the sun with the spectroheliograph ; 

 (3) photography of the spectra of sun-spots and flocculi ; (4) pho- 

 tography of stellar spectra with a grating spectrograph of high dis- 

 persion. Since the Snow telescope has been employed in all of this 

 work, an account of the tests of this instrument should precede a 

 description of the investigations named. 



The Snow telescope was set upon Mount Wilson in January, 1905, 

 but the extremely wet weather of the rainy season rendered it inad- 

 visable to put the mirrors in place until about March 15, 1905. On 

 the return of favorable weather, tests of the telescope were at once 

 undertaken. It was found that the heating of the mirrors by the 

 sun produced marked changes of focal length, together with such 

 evidences of astigmatism as would be expected to follow from the 

 distortion of the plane mirrors. The focal distance lengthened rapidly 

 after the mirrors had been exposed to the solar rays for a short time, 

 but soon reached a maximum, where it remained fairly constant. 

 Speaking approximately (as the results varied considerably at dif- 

 ferent times), the change from normal to maximum, with the mirror 

 of 60 feet focal length, was from 3 inches to 1 foot. It was soon 

 noticed that the change became more marked as the silvered surfaces 

 grew older and more tarnished, on account of the greater absorption 

 of heat. For a time it seemed probable that the use of the telescope 



