MOUNT WILSON OBSERVATORY. 



235 



around the sun, conditions never approximated in regular work, to 

 cause a measurable decrease in the line displacement at the limb, and 

 that this large proportion of superposed sky-light produces no differen- 

 tial effect between strong and weak lines. 



WAVE-LENGTHS IN SPECTROGRAMS OF VENUS. 



Evershed has suggested, as an explanation of his observations, that 

 the wave-lengths in light reflected by Venus vary with the relative 

 positions of Venus, the Sun, and the Earth. 



Several spectrograms of Venus, with an average exposure of one 

 hour each, have been secured at Mount Wilson by Mr. St. John and 

 Mr. Nicholson in the first order of the 18-foot grating spectrograph, 

 using the iron-arc for comparison. Wave-lengths of 43 lines in the 

 nighborhood of X4500 in the spectrum of Venus have been compared 

 with those of the same lines in the spectrum of the sky and of the center 

 of the sun. Measures of 18 sky plates and 9 solar plates show no dif- 

 ference between wave-lengths in the spectra of these two sources. 



Twenty-six of the Venus plates have been measured, for which the 

 angle Venus-Sun-Earth changed from 102° to 32°. The spectra taken 

 with Venus at an altitude of 30° give approximately the same wave- 

 lengths as the sky spectra, but for lower altitudes the wave-lengths of 

 Venus are shorter, irrespective of the relative positions of Venus, the 

 Sun, and the Earth. As it was possible that at low altitudes atmos- 

 pheric dispersion might produce a non-uniform illumination of the slit, 

 the spectrograph was rotated 180° between exposures. The following 

 table gives a brief summary of the present results. Since the dispersion 

 was 3 A per mm., these residuals are near the limit of measurement. 



The observed variation in wave-length may be due to some unelimi- 

 nated instrumental error connected with the low altitude of Venus. 

 At any rate, it will be necessary to secure more plates with Venus on 

 the other side of the sun before the observations can be definitely 

 interpreted. 



WAVE-LENGTHS OF ATMOSPHERIC ABSORPTION LINES. 



Some observations by Perot seem to show a variation of the wave- 

 lengths of the absorption lines of oxj^gen with the altitude of the sun 

 which would indicate a recession of the absorbing centers with a velocity 



