THE PREDECESSORS OF COPERNICUS. 333 



invented and provided with a micrometer it became possible to fix 

 star-places to within about one second of are (1"). Tycho's observing 

 science, perfect in his day — incapable of further improvement — was 

 no more than a rude approximation to Bradley, Astronomer-Eoyal of 

 England in 1750. Bradley's tests were at least sixty times more 

 delicate (l'=:60"). Examples of this sort show how theories are 

 held. 'Certain tests are now available — tests of a certain delicacy. 

 When phenomena can be predicted beforehand as well as they can be 

 subsequently observed, science is perfect up to that point. Increase 

 the delicacy of the tests and a new standard is set up. Wave-motion 

 was pretty well understood at the end of the nineteenth century until 

 the X-rays came and refused, at first, to be reflected, refracted or 

 polarized. 



We in our day have learned a patient tolerance of opinion; wait, 

 these theories that seem so baseless may, perhaps, come to something, 

 as others have done in the past. To what especial and peculiar merit 

 do we owe this acquired virtue of tolerant patience? It is owed solely 

 to the experience of centuries. We have so often seen the impossible 

 become the plausible, and at last the proved and the practical. Can 

 we justly expect that our frame of mind — the strict result of centuries 

 of experience — should have been the attitude of the doctors of the 

 middle ages? Galileo was a great physicist: would not even he re- 

 quire time to accept our modern cobweb theory of the constitution of 

 matter with its ether, molecules, atoms, electrified and non-electrified 

 half-atoms, ions, dissociation, radio-activity and the like? Centuries 

 of experience have taught us to hold theories lightly even while we 

 are using them for present interpretations of phenomena. What 

 physicist doubts that our present theory of electricity needs a thorough- 

 going revision? And yet, who fails to use it where it can serve even 

 a temporary purpose, foreseeing all the while new interpretations in 

 the future? 



The fundamental necessity in studies like the present is to realize 

 the state of mind of our heroes and of the communities in which they 

 lived. The only data are the words of the books they have left us. 



How to interpret their words m, their sense is the central difficulty ; 

 it is often most misleading to interpret them in our own. 'Do unto 

 others as you would that they should do to you' is a golden rule that 

 has been given in nearly the same words by Aristotle, by Christ and 

 by Confucius; yet by 'others' Christ meant all men; Aristotle meant 

 all the free born men of Greece, not their slaves ; and Confucius meant 

 the virtuous among his countrymen and excluded all wicked men and 

 all foreign barbarians. If we consider what was meant by the words 

 ' citizen, ' ' honor, ' ' duty, ' in ancient Eome ; in the later Eoman Empire ; 

 in Constantinople; in the free towns of Italy; in the England of the 



