156 ANNUAL. KEPORT SMITHSONIAN INSTITUTION, 1935 



is very intensely absorbed, a layer of the gas, at its worst, being as 

 opaque as one of metal of equal mass per square centimeter. For 

 water vapor the main absorption bands lie far in the infrared, and 

 are very strong — those with which we are now concerned involve 

 high harmonics of the fundamental vibrations. The coeflicient of 

 absorption, and the intensity of the bands, diminishes rapidly with 

 increasing order of the harmonics and diminishing wave length. 



The oxygen bands are produced by a " forbidden " transition 

 within the molecule, for which the probability of absorption is ex- 

 ceedingly small. This is why the whole mass of oxygen above our 

 heads (equivalent to a layer 2 kilometers thick at standard tempera- 

 ture and pressure) produces absorption lines no stronger than the 

 sodium vapor in a Bunsen flame an inch thick, which contains but 

 a minute percentage of the vapor of the metal. The principal bands 

 of oxygen, in the ultraviolet beyond A1800, are so strong that light 

 of shorter wave length cannot be observed at all in air. The experi- 

 menter must put his whole spectroscope in a gastight case, and ftump 

 it out to an almost perfect vacuum. 



In the visible spectrum, the portions cut out by oxygen or water 

 vapor are very small in extent; but they come exactly in the wrong 

 place — in other words, they hide, line for line, absorption by these 

 same gases which might be produced in the atmosphere of a planet. 



If the planet's atmosphere was decidedly richer in either constitu- 

 ent than the earth's, we might detect the fact, for the lines in the 

 planet's spectrum would be stronger than in that of the moon. Com- 

 parisons of this sort, however, must be made with great precautions. 

 The moon and planet must be at the same altitude when the observa- 

 tions are made (to get equal air-paths). It is not safe, either, to 

 observe the planet early in the evening and wait till the moon rises 

 to the same height, for a change in temperature may have caused 

 the precipitation of water out of the air, though the oxygen, of 

 course, remains the same. With sufficient patience, a time may be 

 found when planet and moon can be seen together, at equal altitudes, 

 and observed almost simultaneously, with the same instrument. 



Early observations of this sort were supposed to show the presence 

 of oxygen and water vapor on Venus and Mars ; but the careful and 

 accurate work of Campbell, in 1894, led him to the conclusion that 

 there was no perceptible difference in the strength of the bands in 

 the two cases, and hence that the amounts of these important 

 substances, above the visible surfaces of either planet, did not exceed 

 one-fourth of those above an equal area of the Earth's. 



A more delicate and very ingenious test was invented, independ- 

 dently, by two distinguished American observers, Lowell and Camp- 

 bell. Wlien Mars (or Venus) is approaching us, or receding, most 

 rapidly, the lines in its spectrum are displaced by the Dojppler shift, 



