OBSERVATION AND EXPERIMENT 71 



in determining how much of the universe may be safely left 

 out of account. Thus the method of approximating to a 

 knowledge of the laws of nature is somewhat like the method 

 of infinite series so much used by mathematicians in numer- 

 ical calculations ; and as it is a condition of success in the 

 use of such series that they be convergent rather than divergent, 

 so is it an essential of scientific sanit}* that the mind be re- 

 stricted by observed facts rather than diverted by pleasing 

 fancies. 



The prime characteristic of the kind of knowledge that leads 

 up to science is its dependence on facts which are permanent 

 and hence verifiable. In the course of the progress of our race 

 there have been certain lumiinous epochs during which observ- 

 ers and experimentalists have revealed more or less of such 

 knowledge. These epochs have been followed, generally, by 

 others of camparative dullness, or positive darkness, during 

 which fact has been replaced by fancy and what is permanent 

 and verifiable has been eclipsed by what is ephemeral and illu- 

 sory. It is my purpose to-night to recall some of the principal 

 events of these epochs, and to enforce, as well as I may, the 

 great lesson they seem to teach us, namely, that science can be 

 maintained only, and can be advanced only, by a constant ap- 

 peal to observation and experiment. 



As we look out on the universe about us the most striking 

 phenomena visible are those which belong to what Galileo and 

 his successors have fitly called " the system of the world." The 

 rising and setting of the sun and moon ; the majestic procession 

 of the seasons ; the splendid array of the stars in the heavens ; 

 the ebb and flow of the sea ; and the nev^er-ending variety from 

 wind and weather, need only to be mentioned to enable us to 

 understand why astronomy is at once the oldest and one of the 

 most highly developed of the sciences. No classes of phenom- 

 ena are so obvious, so omnipresent, and so enduring. They 

 have furnished the symbols of continuity and permanence for 

 all languages in all historic times. The ''fixed stars," for ex- 

 ample, are in fact, as well as in fiction, our standards of refer- 

 ence in the reckoning of time and space ; for are not " Sirius 



