For Dr. Slipher was at no less care to determine just what precision was possible in the case, although a speed corresponding to a spin of twenty-four hours on a globe the size of Venus is well known to be spectroscopically measurable. It would mean a motion toward us of one thousand miles an hour, or about a third of a mile a second. The tilt occasioned by this speed is well within the spectroscope’s ability to disclose. Not content with this, however, by two special investigations, he proved the spectroscope’s actual limits of performance to be far within the quantity concerned. One of them was the determination by the same means and in like manner of the rotation time of Mars, the length of that planet’s day, which in other ways we know to the hundredth of a second, and which is 24ʰ 37ᵐ 23.66ˢ Now Mars offers a test nearly twice as difficult as Venus, even supposing the apparent disks of the two the same, because his diameter being less in the proportion roughly of one-half, the actual speed of a particle at his edge is less for the same time of rotation in the like proportion, and it is only with the speed in miles, not in angular amount, that the spectroscope is concerned. Nevertheless, when a like number of plates were tried on him, they indicated on measurement a rotation time within an hour of the true. This corresponds to half an hour on Venus. We see, therefore, that had Venus’ day been anywhere in the neighborhood of twenty-four hours, Dr. Slipher’s investigation would have disclosed it to within thirty-one minutes. [Illustration: SPECTROGRAM OF JUPITER, GIVING THE LENGTH OF ITS DAY BY THE TILT OF ITS SPECTRAL LINES—V. M. SLIPHER, LOWELL OBSERVATORY.] This result was further borne out by a similar test made by him of Jupiter. Inasmuch as the diameter of Jupiter is twelve times that of Venus, while the rotation time is 9ʰ 50.4ᵐ at the equator, the precision attained on Venus should here have been about a minute. And this is what resulted. Slipher found the rotation time spectrographically 9ʰ 50ᵐ, or in accordance with the known facts, while previous determinations with the spectroscope had somehow fallen short of it. The care at Flagstaff with which the possibility of error was sought to be excluded in this investigation of the length of Venus’ day and the concordant precision in the results are worthy of notice. For it is by thus being particular and systematic that the accuracy of the determinations made there, in other lines besides this, has been secured.