In 1868 Huggins[6] succeeded in thus measuring the velocities of stars in the direction of the line of sight. In 1873 Vogel[7] compared the spectra of the sun’s East (approaching) limb and West (receding) limb, and the displacement of lines endorsed the theory. This last observation was suggested by Zöllner. FOOTNOTES: [1] In the _Encyclopaedia Britannica_, article “Telescope,” and in Grant’s _Physical Astronomy_, good reasons are given for awarding the honour to Lipperhey. [2] Will the indulgent reader excuse an anecdote which may encourage some workers who may have found their mathematics defective through want of use? James Gregory’s nephew David had a heap of MS. notes by Newton. These descended to a Miss Gregory, of Edinburgh, who handed them to the present writer, when an undergraduate at Cambridge, to examine. After perusal, he lent them to his kindest of friends, J. C. Adams (the discoverer of Neptune), for his opinion. Adams’s final verdict was: “I fear they are of no value. It is pretty evident that, when he wrote these notes, _Newton’s mathematics were a little rusty_.” [3] _R. S. Phil. Trans_. [4] The experiment had been made before by one who did not understand its meaning;. But Sir George G. Stokes had already given verbally the true explanation of Frauenhofer lines. [5] _Abh. d. Kön. Böhm. d. Wiss_., Bd. ii., 1841-42, p. 467. See also Fizeau in the _Ann. de Chem. et de Phys_., 1870, p. 211. [6] _R. S. Phil. Trans_., 1868. [7] _Ast. Nach_., No. 1, 864. BOOK IV. THE PHYSICAL PERIOD We have seen how the theory of the solar system was slowly developed by the constant efforts of the human mind to find out what are the rules of cause and effect by which our conception of the present universe and its development seems to be bound. In the primitive ages a mere record of events in the heavens and on the earth gave the only hope of detecting those uniform sequences from which to derive rules or laws of cause and effect upon which to rely. Then came the geometrical age, in which rules were sought by which to predict the movements of heavenly bodies. Later, when the relation of the sun to the courses of the planets was established, the sun came to be looked upon as a cause; and finally, early in the seventeenth century, for the first time in history, it began to be recognised that the laws of dynamics, exactly as they had been established for our own terrestrial world, hold good, with the same rigid invariability, at least as far as the limits of the solar system. Throughout this evolution of thought and conjecture there were two types of astronomers—those who supplied the facts, and those who supplied the interpretation through the logic of mathematics. So Ptolemy was dependent upon Hipparchus, Kepler on Tycho Brahe, and Newton in much of his work upon Flamsteed.