MOUNT WILSON OBSERVATORY. 191 



mainly for the purpose of determining the transmission of the apparatus and 

 of the earth's atmosphere. His results lead to the following conclusions : 



(1) The transmission of the water-cell used in the apparatus is sensibly a 

 linear function of the air-mass, varjdng from 68.3 per cent for air-mass 7 to 

 72.8 per cent for air-mass unity. 



(2) The logarithm of the intensity of the transmitted radiation when 

 plotted as ordinate against air-mass gives a line slightly concave to increasing 

 ordinates, a result in agreement with that found by Abbot from pyrheUometer 

 observations. 



(3) The corrections to the absorption of the water-cell for selective absorp- 

 tion at the silvered surfaces of the telescope are neghgible for stars of spectral 

 type as early as the sun. 



(4) The solar radiation in the region of short wave-lengths is greater than 

 that of Capella, a star of the same spectral type. This result is in agreement 

 with that known from direct comparisons of spectra which show that dwarf 

 stars are bluer than giant stars of the same type. 



(5) The consistency with which observations made with the vacuum thermo- 

 couple can be repeated, the accuracy with which they can be read, and the 

 possibility of self-registration suggest the use of this instrument for measuring 

 the solar constant. 



No radiation work on sun-spots has been attempted, owing to the lack of 

 spots of sufficient size. 



FOCAL CHANGES PRODUCED BY TEMPERATURE CHANGES IN CCELOSTAT 



MIRRORS. 



Further observations have been made by Pettit of the focal changes pro- 

 duced by the coelostat mirrors in the tower and the Snow horizontal telescopes. 

 It was found that of the seven pairs of plane mirrors used, made of crown glass, 

 pyrex glass, and speculum metal, all became concave when exposed to the 

 sun, with the exception of the speculum mirrors and the crown-glass mirrors of 

 the Snow telescope. This result is difficult of explanation on the present 

 theory of the expansion of mirrors. No material difference was found when a 

 concave mirror in place of a lens was used to project the image. The most 

 rapid changes of focus were observed in the case of crown-glass mirrors 12 

 inches in diameter and 1 inch thick. In this case the change of focus at the 

 150-foot tower telescope amounted to 5 meters in 3^ hours. The smallest 

 changes were those with the speculum and pyrex-glass mirrors. With the 

 latter the focus changed 55 cm. during the first hour, but thereafter remained 

 essentially constant during observations extending over 8 hours. 



RESEARCHES ON THE MOON AND PLANETS. 

 THE ROTATION PERIOD OF VENUS. 

 Reference was made in the last annual report to the possibihty of using 

 the spectrograms of Venus obtained at Mount Wilson in a study of the 

 rotation period of the planet and a possible determination of the solar par- 

 allax. St. John and Nicholson have completed the necessary measurements 

 and have studied them with reference to these objects. The change in 

 wave-length of lines at the center of the illuminated portion of Venus is a 

 function of the rotation of the planet and the relative orbital velocity of 

 Venus and the earth. Rotation of the planet would produce changes of 

 wave-length which, in the comparatively short interval over which observa- 



