Dr. Slipher chose his time astutely. He selected the occasion when Venus was passing through superior conjunction, or the point in her orbit as regards us directly beyond the sun. At first sight this might seem to be the worst as well as the most impracticable of epochs, inasmuch as the planet is then not only at her farthest from the Earth, but in a line with the Sun, and so drowned in his glare. But in point of fact any tilt of the spectral lines is then, owing to phase, twice what it is at elongation, and exceeds still more what it is when Venus has her greatest lustre [see NOTE 3]. In his purpose he was abetted by the Flagstaff air, which enabled the planet to be spectrographed much nearer the sun than would otherwise have been the case. He thus selected the best possible opportunity. To guard against any subsequent bias on the part of the examiner of the plates, after the spectroscope had taken a plate it was then reversed, and the process repeated on another one, the iron being sparked as before. What had been the right side of Venus with regard to the red end of the spectrum thus became the left one, and _vice versa_. In this manner, when the plates came to be measured for tilt, the measurer would have no indication from the spectrum itself which way the lines might be expected to tilt; he could, therefore, not be influenced either consciously or unconsciously in his decision. [Illustration: SPECTROGRAM OF VENUS, SHOWING ITS LONG DAY—V. M. SLIPHER, LOWELL OBSERVATORY, 1903.] Eight plates with their comparison ferric spectra were thus secured; four with the spectroscope direct, four with it reversed. They were then shuffled, their numbers hidden, and given to Dr. Slipher to measure. The spectral lines told their own story, and without prompting. All the plates agreed within the margin of error accordant with their possible precision, a precision some thirty times that of Belopolski’s experiment on the same lines,—a result not derogatory of that investigator, but merely illustrative of superior equipment. They showed conclusively that a rotation of anything like twenty-four hours was out of the question. They yielded, indeed, testimony to a negative rotation of three months, which, interpreted, means that so slow a spin as this was beyond their power to precise.