number of foci, which prevent a clear view from being obtained at any point.

What astonishes the reader of the Opticks is the persistence with which Newton follows out the consequences of a preconceived theory, and tests the one notion by a wonderful variety of simple comparisons with fact. The ease with which he invents new combinations, and foresees the results, subsequently verified, produces an insuperable conviction in the reader that he has possession of the truth. And it is certainly the theory which leads him to the experiments, most of which could hardly be devised by accident. Newton actually remarks that it was by mathematically determining all kinds of phenomena of colours which could be produced by refraction that he had "invented" almost all the experiments in the book, and he promises that others who shall "argue truly," and try the experiments with care, will not be disappointed in the results.1

The philosophic method of Huyghens was the same as that of Newton, and Huyghens' investigation of double refraction furnishes almost equally beautiful instances of theory guiding experiment. So far as we know double refraction was first discovered by accident, and was described by Erasmus Bartholinus in 1669. The phenomenon then appeared to be entirely exceptional, and the laws governing the two paths of the refracted rays were so unapparent and complicated, that Newton altogether misunderstood the phenomenon, and it was only at the latter end of the last century that scientific men began to comprehend its laws.

Nevertheless, Huyghens had, with rare genius, arrived at the true theory as early as 1678. He regarded light as an undulatory motion of some medium, and in his Traité de la Lumière he pointed out that, in ordinary refraction, the velocity of propagation of the wave is equal in all directions, so that the front of an advancing wave is spherical, and reaches equal distances in equal times. But in crystals, as he supposed, the medium would be of unequal elasticity in different directions, so that a disturbance would reach unequal distances in equal times, and the wave produced would have a spheroidal form.

Opticks, bk. i. part ii. Prop. 3. 3rd ed. p. 115.

Huyghens was not satisfied with an unverified theory. He calculated what might be expected to happen when a crystal of calc-spar was cut in various directions, and he says: “I have examined in detail the properties of the extraordinary refraction of this crystal, to see if each phenomenon which is deduced from theory would agree with what is really observed. And this being so, it is no slight proof of the truth of our suppositions and principles; but what I am going to add here confirms them still more wonderfully; that is, the different modes of cutting this crystal, in which the surfaces produced give rise to refraction exactly such as they ought to be, and as I had foreseen them, according to the preceding theory."

Newton's mistaken corpuscular theory of light caused the theories and experiments of Huyghens to be disregarded for more than a century; but it is not easy to imagine a more beautiful or successful application of the true method of inductive investigation, theory guiding experiment, and yet wholly relying on experiment for confirmation.

Candour and Courage of the Philosophic Mind.

Perfect readiness to reject a theory inconsistent with fact is a primary requisite of the philosophic mind. But it would be a mistake to suppose that this candour has anything akin to fickleness; on the contrary, readiness to reject a false theory may be combined with a peculiar pertinacity and courage in maintaining an hypothesis as long as its falsity is not actually apparent. There must, indeed, be no prejudice or bias distorting the mind, and causing it to pass over the unwelcome results of experiment. There must be that scrupulous honesty and flexibility of mind, which assigns adequate value to all evidence; indeed, the more a man loves his theory, the more scrupulous should be his attention to its faults. It is common in life to meet with some theorist, who, by long cogitation over a single theory, has allowed it to mould his mind, and render him incapable of receiving anything but as a contribution to the truth of his one theory. A narrow and intense course of thought may sometimes lead to great results, but the adop tion of a wrong theory at the outset is in such a mind irretrievable. The man of one idea has but a single chance of

truth. The fertile discoverer, on the contrary, chooses between many theories, and is never wedded to any one, unless impartial and repeated comparison has convinced him of its validity. He does not choose and then compare; but he compares time after time, and then chooses.

Having once deliberately chosen, the philosopher may rightly entertain his theory with the strongest fidelity. He will neglect no objection; for he may chance at any time to meet a fatal one; but he will bear in mind the inconsiderable powers of the human mind compared with the tasks it has to undertake. He will see that no theory can at first be reconciled with all objections, because there may be many interfering causes, and the very consequences of the theory may have a complexity which prolonged investigation by successive generations of men may not exhaust. If, then, a theory exhibit a number of striking coincidences with fact, it must not be thrown aside until at least one conclusive discordance is proved, regard being had to possible error in establishing that discordance. In science and philosophy something must be risked. He who quails at the least difficulty will never establish a new truth, and it was not unphilosophic in Leslie to remark concerning his own inquiries into the nature of heat

"In the course of investigation, I have found myself compelled to relinquish some preconceived notions; but I have not abandoned them hastily, nor, till after a warm and obstinate defence, I was driven from every post." 1

Faraday's life, again, furnishes most interesting illustrations of this tenacity of the philosophic mind. Though so candid in rejecting some theories, there were others to which he clung through everything. One of his favourite notions resulted in a brilliant discovery; another remains in doubt to the present day.

The Philosophic Character of Faraday.

In Faraday's researches concerning the connection of magnetism and light, we find an excellent instance of the pertinacity with which a favourite theory may be pursued,

1 Experimental Inquiry into the Nature of Heat. Preface, p. xv.

so long as the results of experiment do not clearly negative the notions entertained. In purely quantitative questions, as we have seen, the absence of apparent effect can seldom be regarded as proving the absence of all effect. Now Faraday was convinced that some mutual relation must exist between magnetism and light. As early as 1822, he attempted to produce an effect upon a ray of polarised light, by passing it through water placed between the poles of a voltaic battery; but he was obliged to record that not the slightest effect was observable. During many years the subject, we are told, rose again and again to his mind, and no failure could make him relinquish his search after this unknown relation. It was in the year 1845 that he gained the first success; on August 30th he began to work with common electricity, vainly trying glass, quartz, Iceland spar, &c. Several days of labour gave no result; yet he did not desist. Heavy glass, a transparent medium of great refractive powers, composed of borate of lead, was now tried, being placed between the poles of a powerful electro-magnet while a ray of polarised light was transmitted through it. When the poles of the electro-magnet were arranged in certain positions with regard to the substance under trial, no effects were apparent; but at last Faraday happened fortunately to place a piece of heavy glass so that contrary magnetic poles were on the same side, and now an effect was witnessed. The glass was found to have the power of twisting the plane of polarisation of the ray of light.

All Faraday's recorded thoughts upon this great experiment are replete with curious interest. He attributes his success to the opinion, almost amounting to a conviction, that the various forms, under which the forces of matter are made manifest, have one common origin, and are so directly related and mutually dependent that they are convertible. "This strong persuasion," he says, " extended to the powers of light, and led to many exertions having for their object the discovery of the direct relation of light and electricity. These ineffectual exertions could not remove my strong persuasion, and I have at last suc

2

Bence Jones, Life of Faraday, vol. i. p. 362. 2 Ibid. vol. ii. p. 199.

ceeded." He describes the phenomenon in somewhat figurative language as the magnetisation of a ray of light, and also as the illumination of a magnetic curve or line of force. He has no sooner got the effect in one case, than he proceeds, with his characteristic comprehensiveness of research, to test the existence of a like phenomenon in all the substances available. He finds that not only heavy glass, but solids and liquids, acids and alkalis, oils, water, alcohol, ether, all possess this power; but he was not able to detect its existence in any gaseous substance. His thoughts cannot be restrained from running into curious speculations as to the possible results of the power in certain cases. "What effect," he says, "does this force have in the earth where the magnetic curves of the earth traverse its substance? Also what effect in a magnet?" And then he falls upon the strange notion that perhaps this force tends to make iron and oxide of iron transparent, a phenomenon never observed. We can meet with nothing more instructive as to the course of mind by which great discoveries are made, than these records of Faraday's patient labours, and his varied success and failure. Nor are his unsuccessful experiments upon the relation of gravity and electricity less interesting, or less worthy of study.

Throughout a large part of his life, Faraday was possessed by the idea that gravity cannot be unconnected with the other forces of nature. On March 19th, 1849, he wrote in his laboratory book," Gravity. Surely this force must be capable of an experimental relation to electricity, magnetism, and the other forces, so as to bind it up with them in reciprocal action and equivalent effect?"1 He filled twenty paragraphs or more with reflections and suggestions, as to the mode of treating the subject by experiment. He anticipated that the mutual approach of two bodies would develop electricity in them, or that a body falling through a conducting helix would excite a current changing in direction as the motion was reversed. "All this is a dream," he remarks; "still examine it by a few experiments. Nothing is too wonderful to be true, if

1 See also his more formal statement in the Experimental Researches in Electricity, 24th Series, § 2702, vol. iii. p. 161.