[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.
When Galileo directed his telescope to the heavens, when Secchi and Huggins studied the chemistry of the stars by means of the spectroscope, and when Warren De la Rue set up a photoheliograph at Kew, we see that a progress in the same direction as before, in the evolution of our conception of the universe, was being made. Without definite expression at any particular date, it came to be an accepted fact that not only do earthly dynamics apply to the heavenly bodies, but that the laws we find established here, in geology, in chemistry, and in the laws of heat, may be extended with confidence to the heavenly bodies. Hence arose the branch of astronomy called astronomical physics, a science which claims a large portion of the work of the telescope, spectroscope, and photography. In this new development it is more than ever essential to follow the dictum of Tycho Brahe—not to make theories until all the necessary facts are obtained. The great astronomers of to-day still hold to Sir Isaac Newton’s declaration, “Hypotheses non fingo.” Each one may have his suspicions of a theory to guide him in a course of observation, and may call it a working hypothesis. But the cautious astronomer does not proclaim these to the world; and the historian is certainly not justified in including in his record those vague speculations founded on incomplete data which may be demolished to-morrow, and which, however attractive they may be, often do more harm than good to the progress of true science. Meanwhile the accumulation of facts has been prodigious, and the revelations of the telescope and spectroscope entrancing.