526 THE POPULAR SCIENCE MONTHLY 



each according to its kind, into a spectral image band which may then 

 be viewed by the eye or caught upon a photographic plate. 



One of the interesting applications of the spectroscope lies in its 

 ability to detect motion in the line of sight, or in just the direction in 

 which the eye can not. 



It was reasoned by Doppler in 1842 that if an object be coming 

 toward the observer emitting light as it does so, each wave-length of its 

 spectrum should be shortened in proportion to the relative speed of its 

 approach as compared with the speed of light, because each new wave 

 is given out nearer the observer than would otherwise be the case and 

 its wave-length thus seemingly decreased. Eeversely it will be length- 

 ened if the object be receding from the observer or he from it. Tbis 

 would change the color of each wave-length and so of the object, were 

 it not that while each hue moves into the place of the next, like the 

 guests at Alice's tea-party in Wonderland, some red rays pass off the 

 visible spectrum, but new violet rays come up from the infra violet and 

 the spectrum is as complete as before. This unfortunate infecundity of 

 his principle in Doppler's own hands was remedied in 1848 by Fizeau, 

 who pointed out that the dark lines in the spectra can be used as meas- 

 ures of the shift. In all spectra are gaps where individual wave- 

 lengths are absorbed or omitted and these, the lines in the spectrum, 

 tell the tale. 



This principle is applicable not only to a body moving as a whole, 

 but to differing motions of its parts if the body be large enough to 

 show a disk. Now, if a body be rotating, one side of it will be ap- 

 proaching the observer, while the opposite side is receding from him, 

 and if the slit be placed perpendicular to the axis about which the spin 

 takes place, each spectral line will appear not straight across the spec- 

 trum of the object, but skewed, the approaching side being tilted to the 

 violet end, the receding side to the red. 



This principle was put in practise by one of the observatory staff, 

 Dr. Slipher, to determine spectrographically the rotation of Venus. By 

 placing the slit parallel to the ecliptic or, more property, to the orbit of 

 Venus, which is practically the same thing, it would find itself along 

 what we have reason to suppose the equator of the planet and thus by 

 its tilt give evidence of the rotation period capable of measurement. 

 Even a considerable error in the position of the equator would make 

 little difference in the rotational result. In order that there might be 

 no question of illusion or personal bias, photographs instead of eye 

 observations of the spectrum were made. 



Dr. Slipher began by considering the take off before he jumped. 

 His sagacity greatly influenced the result. It might seem as if the best 

 time to examine the planet for rotation were when it is farthest from 

 the sun and is best seen. This indeed was the time selected by 

 Belopolsky, who examined Venus spectrographically between the time 



