MOUNT WILSON OBSERVATORY. 219 



land's scale. Investigations of the intensity of the lines just visible on the 

 spectrograms of Venus indicate that the equivalent of a depth of 9.2 meters 

 of oxygen under conditions corresponding to those in the laboratory should 

 give visible lines. Since in these observations of Venus the path of the light 

 is several times the radial depth of the atmosphere, ranging from 5.2 to 7.2 

 in some cases, it follows that the absence of absorption lines means that above 

 the level at which reflection takes place there is less than the equivalent of 1, 

 or at most 2, meters of oxygen capable of absorbing radiation in the same way 

 as gas in the laboratory. The oxygen in the earth's atmosphere is equivalent 

 to a column 1,500 meters in depth, so that the oxygen in the portions of the 

 planet's atmosphere accessible to observation can not be more than one one- 

 thousandth that of the terrestrial atmosphere. From similar considerations 

 it may be shown that there is less than 1 mm. of precipitable water in the 

 layer of the planet's atmosphere traversed by the solar beam. It is proposed 

 to extend the observations to the band near X7200, since the lines of this band 

 can be produced by smaller quantities of water-vapor than those of shorter 

 wave-length. 



FOCAL CHANGES PRODUCED BY TEMPERATURE CHANGES IN CCELOSTAT 



MIRRORS. 



Mr. Pettit has made a number of tests of the effect of the sun's heat upon 

 the ccelostat mirrors of the Snow telescope and the two tower telescopes in 

 producing changes of focal length. In the case of the Snow telescope the 

 glass mirrors were tested and then replaced by speculum mirrors. It was 

 found that with the speculum the amount of change was only two-fifths as 

 great as with the glass, and that a constant focus was reached in about 1 hour 

 as against 1| hours for the glass mirrors. A comparison was also made 

 between the speculum mirrors in the Snow telescope and the glass mirrors of 

 the 60-foot tower telescope. The latter showed much greater ranges of focus, 

 which continued over a decidedly longer period of time. 



An investigation of the focal changes of the 150-foot tower telescope showed 

 that the effect of the liquid circulating system around the mirrors is to reduce 

 the amount of change by about one-third of its value. The time required to 

 reach a constant focus is about 7 hours as against 4^ hours without the cool- 

 ing system. During this time the change appears to be nearly linear. In 

 February a test was made after replacing the usual mirrors with two 12-inch 

 pyrex-glass mirrors, 2 inches in thickness. The changes in focus were found 

 to be very greatly reduced, being only about two-fifths as great for the pyrex 

 mirrors without the cooling system as for the ordinary mirrors with the sys- 

 tem in operation. The time required for the pyrex mirrors to reach a con- 

 stant focus was found to be only 50 minutes, and no tendency to astigmatism 

 was noted during observations extending over the succeeding 7 hours. 



DISTRIBUTION OF LIGHT IN THE SOLAR CORONA. 



Mr. Pettit has applied the registering microphotometer to the measure- 

 ment of the distribution of light in the corona, using for this purpose photo- 

 graphs obtained by him at the eclipse of 1918 at Matheson, Colorado, with 

 exposure times ranging from 1 to 40 seconds. A turntable equal in diameter 

 to that of the sun's image was made to rotate beneath the microscope objec- 

 tive in synchronism with the linear travel of the registering apparatus, and, 

 by means of a cross-slide on the microscope stage, the limb of the sun was set 



