Physical and mathematical sciences. 321

himself was disposed to lean to the hypothesis ofundulations.* In the Optics, he most cautiously urgesthe reader to attach no physical conception to the theoryof Jits. He certainly employed the molecular theoryfor the purpose of calculating the results mathematically,especially of establishing the law of refraction. But, asProfessor Airey has pointed out, there was a sufficientreason for this, in the circumstance that here he hadthe mathematical methods ready established : in thetheory of the undulations, the first principle of suchinvestigations was as yet unknown.t

Newton’s System of the World. — History of the. Discovery.

In 1666, the plague had broken out in Cambridge,which drove Newton into the country. Retiring toWoolsthorpe, he devoted himself to philosophical con.templations. As it is the characteristic of vulgar mindsto attach the greatest importance whatever is un-common, astonishing, and marvellous, though often quiteundeserving of any particular attention, so it is themark of a powerful mind to perceive, in things of com-mon occurrence, how much is to be learnt; and to deduceimportant reflections from circumstances which thegenerality of men pass by unheeded. Mankind hadbeen for ages accustomed to witness the fact that allbodies fall directly to the ground the moment supportis withdrawn. But no one ever saw in it any thingwonderful. To Newton , however, apples falling, as itwere, spontaneously from the trees in the orchard, atWoolsthorpe, afforded a topic of reflection, and led himinto a train of profound thought, from which, ultimately,resulted nothing less than a complete discovery of thesystem of the world.

This power or force, he observed, extends to somedistance from the surface of the earth : it exists at the

* See Dr. Young’s paper, Phil. Trans. 1802.f Journal of Science, June, 1883.

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