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-Royal of England in 1750. Bradley's tests were at least sixty times more delicate (1′ = 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 require 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 thoroughgoing 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 in 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 Rome; in the later Roman Empire; in Constantinople; in the free towns of Italy; in the England of the
