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. Reversely it will be lengthened if the object be receding from the observer or he from it. This 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 measures of the shift. In all spectra are gaps where individual wavelengths 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 approaching 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 spectrum 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 properly, 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
