THE PLANET VENUS 529 



Another favorable circumstance accrued to his choice of time, the 

 then diminutive disk she showed. One might suppose at first that this 

 would be a drawback. But the exact reverse is the case. Provided a 

 planet shows any appreciable disk, the smaller that disk the better for 

 spectrographs rotation purposes. For it is the actual velocity that 

 registers itself upon the plate, in the displacement of the several parts 

 of the spectral lines. The greater the diameter, therefore, for a given 

 rotational velocity, the smaller the angular tilt, and reversely, the 

 smaller the diameter the greater the seeming inclinations of the lines. 

 The effect is very striking in the case of Jupiter. 



By his choice, then, he began by securing the most favorable con- 

 ditions for disclosing a tilt if any existed, and for thus revealing a 

 parallel, in which condition they fall upon the prism. This spreads 

 them out into the spectrum and another small telescope focuses them, 

 short period rotation if such actually were the fact. 



After the spectrum of Venus had been secured on a plate there was 

 taken on either side of it for reference and check the spectrum of iron 

 by means of a spark sent through a tube containing its vapor. As the 

 iron-vapor was stationary with regard to the observer, its lines were 

 necessarily without motional tilt. When a plate had thus been made, 

 the photographic part of the instrument was reversed, the camera be- 

 ing placed above the collimator instead of below it, and the process 

 repeated. This had the effect of reversing the tilt in the resulting 

 spectrogram as compared with the first one. Still other checks were 

 used by Dr. Slipher, such as placing the slit in some cases perpendicular 

 instead of parallel to the supposed equator, which need not be gone into 

 here. They all proved corroborative of his final result. 



Fifteen plates with their comparison spectra of iron were thus ob- 

 tained : seven with the camera above, eight with it below. To prevent 

 any bias on his part, Dr. Slipher had the numbers on the plates con- 

 cealed and the plates handed him for measurement without his knowl- 

 edge of which was which. The plates were thus made to tell their own 

 story without prompting or prejudice. The story they told they agreed 

 in within the margin of error accordant with their possible precision. 

 It was : that a rotation of twenty-four hours or a^thing like it was out 

 of the question. They yielded, indeed, testimony to a negative rotation 

 of three months, which being interpreted meant that so slow a spin as 

 Venus's was beyond their power to disclose. 



For just what their power to precise might be Dr. Slipher was at no 

 less care to determine. Not content with knowing that on a globe the 

 size of Venus the velocity due to spin in twenty-four hours should be 

 spectroscopically measurable, being actually a thousand miles an hour 

 and by the principle explained above giving spectroscopically a shift of 

 over a mile a second, he determined to test the matter directly by the 

 performance of the spectroscope on Mars. For this purpose seven 



