Applying it to the phenomenon which it is intended to illustrate, if light is being received from a star by an earth, travelling across the direction of the ray, the telescope (which in this case represents the umbrella) must be tilted forward to catch the light. Now on reference to Fig. 4 it will be seen that the earth is travelling across the direction of rays from the star in March and September; and in opposite directions in the two cases. Hence the telescope must be tilted a little, in opposite directions, to catch the light; or, in other words, the star will appear to be farthest south in March, farthest north in September. And so at last the puzzle was solved, and the solution was found, as so often happens, to be of the simplest kind; so simple when once we know, and so terribly hard to imagine when we don't! It may comfort us in our struggles with minor problems to reflect that Bradley manfully stuck to his problem for two or three years. It was probably never out of his thoughts, waking or sleeping; when at work it was the chief object of his labours, and when on a pleasure party he was ready to catch at the slightest clue, in the motion of a wind-vane on a boat, which might help him to the solution. [Sidenote: Results of discovery.] The discovery of aberration made Bradley famous at a bound. Oxford might well be proud of her two Savilian Professors at this time, for they had both made world-famous discoveries--Halley that of the periodicity of comets, and Bradley of the aberration of light. How different their tastes were and how difficult it would have been for either to do the work of the other! Bradley was no great mathematician, and though he was quite able to calculate the orbit of a comet, and carried on such work when Halley left it, it was probably not congenial to him. Halley, on the other hand, almost despised accurate observations as finicking. "Be sure you are correct to a minute," he was wont to say, "and the fractions do not so much matter." With such a precept Bradley would never have made his discoveries. No quantity was too small in his eyes, and no sooner was the explanation of aberration satisfactorily established than he perceived that though it would account for the main facts, it would not explain all. There was something left. This is often the case in the history of science. A few years ago it was thought that we knew the constitution of our air completely--oxygen, nitrogen, water vapour, and carbonic acid gas; but a great physicist, Lord Rayleigh, found that after extracting all the water and carbonic acid gas, all the oxygen and all the nitrogen, there was something left--a very minute residuum, which a careless experimenter would have overlooked or neglected, but which a true investigator like Lord Rayleigh saw the immense importance of. He kept his eye on that something left, and presently discovered a new gas which we now know as argon. Had he repeated the process, extracting all the argon after the