a velocity comparable with that of the water-waves, or no change will be observed. The trains of the second illustration must have a velocity comparable with that of the aërial waves producing sound, or no change of tone will be produced. And in like manner a star or other celestial body must be approaching us, or receding from us, with a velocity comparable with that of the ethereal waves producing light, or no change of color will be produced, the color of light corresponding with the tone of sound. Unfortunately for this purpose, though most fortunately in other respects, light travels at so enormous a rate that even the swiftest motions of the heavenly bodies seem rest by comparison. What, for instance, is the rush of even Newton's comet past its point of nearest approach to the sun, though at the rate of more than 300 miles per second, to the flight of light over nearly 200,000 miles in the same time? "Very much as the movement of a person taking only six steps a minute, each less than half a yard long, to the rush of the swiftest express-train. Yet astronomers have undertaken to measure the approach and recession of stars, moving—in some cases—with less than a tenth part of that comet's motion, and whose velocity, therefore, sinks into still more utter insignificance by comparison with that of light.
Secchi claims (but not justly) to have first invented and applied the method used for this purpose, which consists in noting whether some known line of the spectrum of a heavenly body changes in position—either by moving toward the violet end of the spectrum, which would imply approach, or by moving toward the red end, which would imply recession. Of course, the method is exceedingly delicate and difficult, involving a number of details which would be quite unsuited to these pages; but that, in principle, is its nature. Secchi tried the method, and failed to get any results from it, announcing his unsuccessful attempt in March, 1868. "Then," he says, "Mr. Huggins retried (reprit) the method, announcing in April, 1868, the discovery that Sirius is receding at the rate of twenty miles per second." Secchi should know well, however, that our great spectroscopist did not achieve this success in the few weeks between Secchi's announcement of failure and Huggins's announcement of success. Months had elapsed, during which Huggins had been struggling with this difficult problem. If the enunciation of the method gave claim to the credit of its successful application, I myself could advance a stronger claim than Secchi's, for in an essay in Fraser's Magazine for January, 1868, I definitely indicated the nature and value of the method. But I would rather refer to the circumstance as enabling me to support Huggins's assertion that he was observing by this method for months before Secchi announced his own failure, for immediately on the appearance of my essay I received a letter from Dr. Huggins, mentioning (in confidence, until his paper should be published) that he had been for some time striving for success