June 24, 1910] 



SCIENCE 



991 



oxygen in the Martian atmospliere sliould in- 

 troduce the same absorption lines which are 

 introduced by the earth's atmosphere in the 

 sun-earth spectrum. 



If the distance between Mars and the 

 earth is not changing rapidly, the water 

 vapor and oxygen lines from Mars and the 

 lines from the earth will coincide. When this 

 condition of coincidence exists, it is clearly 

 a difficult problem to detect moderate quanti- 

 ties of water vapor and oxygen in the Mar- 

 tian atmosphere, for the evidence of Martian 

 absorption will be overwhelmed by the ab- 

 sorption of the richly laden terrestrial atmos- 

 phere, especially if the observer be near sea 

 level. To hope for success, the observations 

 should he made from a high-altitude station, 

 at times when the overlying air strata carry 

 a minimum of water vapor, and when the 

 planet is as near the zenith as practicable; 

 observing the lunar spectrum, under identical 

 conditions, for comparison. 



Because of the faintness of the Martian 

 and lunar spectra, it has been found that we 

 are limited to low dispersion in visual obser- 

 vations: and that when the distance between 

 the two planets is constant or nearly so, low 

 dispersion oifers a more sensitive method than 

 high dispersion, even when photography is 

 employed. 



Complying with the conditions in the two 

 preceding paragraphs, the writers photo- 

 graphed the spectra of Mars and the moon 

 last September, from the summit of Mt. 

 Whitney. The conclusion drawn from that 

 investigation was, in brief, that the quantity 

 of any water vapor then existing in the equa- 

 torial atmosphere of Mars was too small to be 

 detected by the spectrographic methods avail- 

 able. This does not mean that the Martian 

 atmosphere was carrying no water vapor, but 

 only that the quantity must have been very 

 small. 



At times other than those when Mars is 

 near opposition, the earth and Mars are rela- 

 passing down, to the planet's surface. On the 

 other hand, the rays did not, on the present occa- 

 sion, pass through the planet's atmosphere at 

 right angles to the strata, but the average angle 

 of incidence and reflexion was about 20°. 



tively approaching or receding from one 

 another. Their relative velocity at quadra- 

 ture may amount to 20 km., more or less, per 

 second, depending upon the concurrence of 

 favorable circumstances. 



When Mr. Campbell was photographing the 

 spectrum of Mars, in December, 1896. with a 

 Eowland grating, fourth order,' 568 lines per 

 mm. (14,438 per inch), he realized that the 

 Doppler-Fizeau principle offers great advan- 

 tages, in theory, for solving the problem of 

 the Martian atmosphere, for on photographs 

 of the spectrum, made near quadrature, with 

 sufficiently high dispersion, the Martian ab- 

 sorption lines and the terrestrial absorption- 

 lines should be separated. At that time 

 (thirteen years ago) the method could not 

 succeed, for all the prominent water vapor 

 and oxygen lines are in the region on the red 

 side of ^ 5875, and the photographic dry 

 plates then available were not sufficiently 

 sensitive to record this region. Even in the 

 fairly sensitive region X 5700-^5800 the 

 grating spectrograms of Mars were underex- 

 posed. The successes of recent years in sensi- 

 tizing dry plates to yellow, orange and red 

 light have encouraged the present effort to 

 apply the method. 



A spectrograph, designed by Mr. Campbell 

 to meet the requirements of the problem and 

 used in connection with the 36-inch refractor, 

 contains an excellent Michelson five-inch 

 plane grating (15,000 lines per inch) which 

 gives a brilliant spectrum in the second order 

 on one side, and this was utilized. The 

 wooden mounting of the spectrograph was de- 

 signed at all points to resist differential flex- 

 ure during the intervals of exposure to the 

 planet and to the moon. The instrument was 

 adjusted, the observations were secured, and 

 the measures and reductions of the spectro- 

 grams weye all made by Mr. Albrecht. 



It was planned to secure observations of 

 Mars and the moon on or near January 17, 

 1910, as the planet was in quadrature at that 

 time. The spectrographic velocity of Mars 

 with reference to the earth was then 18.8 km. 

 per second, recession. Unfavorable weather 

 ^ Asirophysical Journal, 5, 236, 1S97. 



