Thus when we reason upon them we see that the peculiar markings of the planet lose their oddity, becoming the very pattern and prototype of what we should expect to view. Interpreted, they present us the picture of a plight more pitiable even than that of Mercury. For the nearly perfect circularity of Venus’ orbit prevents even that slight change from everlasting sameness which the libration of Mercury’s affords. To Venus the Sun stands substantially stock-still in the sky,—a fact which must prove highly reassuring to Ptolemaic astronomers there, if there be any still surviving from her past. No day, no seasons, practically no year, diversifies existence or records the flight of time. Monotony eternalized,—such is Venus’ lot. What visual observations have thus discovered of the rotation time of Venus, with all that follows from it, the spectroscope at Flagstaff has confirmed. At Dr. Slipher’s hands, spectrograms of the planet have told the same tale as the markings. It was with special reference to this point that the spectrograph there was constructed, and the first object to which it was directed was Venus.[8] [8] Lowell Observatory Bulletin No. 3. The planet’s rotation time was to be investigated by means of the motion it brought about in the line of sight. Visual observation, telescopically, reveals motion thwart-wise by the displacement it produces in the field of view; spectroscopic observation discloses motion to or from the observer by the shift it causes in the spectral lines due to a stretching or shortening of their wave-lengths. The spectroscope is an instrument for analyzing light. Ordinary light consists of light of various wave-lengths. By means of a prism or grating these are dispersed into a colored ribbon or band, the longer waves lying at the red end of the spectrum, as the ribbon is called, the shorter at the violet. Now the spectroscope is primarily such a prism or grating placed between the image and the observer, by means of which a series of colored images of the object are produced. In order that these may not overlap and so confuse one another, the light is allowed to enter the prism only through a narrow slit placed across the telescopic image of the object to be examined. Thus successive images of what is contained by the slit are presented arranged according to their wave-lengths. In practice the rays of light from the slit enter a small telescope called the collimator, and are there rendered parallel, in which condition they fall upon the prism. This spreads them out into the spectrum and another small telescope focusses them, each according to its kind, into a spectral image band which may then be viewed by the eye or caught upon a photographic plate.