248 CARNEGIE INSTITUTION OF WASHINGTON. 



variation with altitude suggests atmospheric refraction, and hence 

 cot h is used as one of the independent variables. Dispersive refrac- 

 tion at altitudes of 5° and 10° displaces the photographic image 

 relatively to the visual image very appreciably. This leads to un- 

 symmetrical illumination of the slit, the degree of asymmetry depend- 

 ing upon the diameter of the image relative to the width of the slit. 

 Twenty-nine plates were secured at the last elongation of Venus 

 with a blue ray-filter in the guiding telescope and more are being taken 

 at the present elongation. Measures of these plates indicate that when 

 the photographic image is symmetrically placed on the slit the dis- 

 crepancies in wave-length disappear. 



When the observations are corrected for the above systematic dis- 

 placements the differences between morning and evening series are 

 less than their probable errors. Consequently they give no indica- 

 tion of a correction to the assumed parallax, 8 ''80, or of a rate of rotation 

 more rapid than that found by Slipher. The high declinations of 

 Venus at the present elongation offer exceptionally favorable condi- 

 tions for further investigations on rotation, as it will be feasible to 

 obtain plates in the red, with the advantages of working with longer 

 wave-lengths and of using the fixed oxygen lines as standards of ref- 

 erence. 



ABSENCE OF OXYGEN AND WATER-VAPOR LINES FROM THE SPECTRUM OF 



VENUS. 



Investigations on the presence of water-vapor and oxygen in plan- 

 etary atmospheres have in the past been made with a dispersion so 

 low that the lines of the atmospheric bands were integrated. Fur- 

 ther, the observations were confined to changes of intensity. The 

 spectrum of Venus on a scale of 3 a per mm. has now been compared 

 by Messrs, St. John and Nicholson with the sky spectrum from 

 X3900 to X6900. These observations were made when the relative 

 velocity of Venus and the earth was so large that lines originating in 

 the atmosphere of Venus should have been completely separated from 

 the lines due to the earth's atmosphere, the relative displacement 

 being 0.25 a. Solar lines of intensity 00 and 000 are present in the 

 spectrograms, but among the lines originating in the atmosphere of 

 Venus there is no trace of the water- vapor lines at X5900 or of the oxy- 

 gen lines in the a and B bands at X6300 and X6900. The only water- 

 vapor and oxygen lines present are those of the terrestrial atmospheric 

 lines. These definitely negative results in the case of Venus will 

 make it of interest to obtain spectrograms of Mars and Jupiter on 

 the same scale. 



