Dollond's achromatic telescope, notwithstanding Newton's dictum that the improvement of telescopes by refraction was desperate.

The prolonged controversy between the supporters of the undulatory and emission theories of light had commenced in the time of Newton, and both sides have claimed the great philosopher as an advocate of their respective views. Hooke was in those days the leading champion of the doctrine of undulations; and though Newton himself framed a hypothesis on a similar basis, he carefully avoided pledging himself to a belief in its truth, and seemed more inclined to account for the phenomena of light by the emission of luminous particles. The great difficulty which staggered Newton in Hooke's undulatory hypothesis, was one that was not removed until a very much later period. If light was caused by a vibrating medium, as Hooke maintained, 'something after the manner that vibrations in the air cause a sensation of sound by beating against the organs of hearing,' how came it that sound, after passing through an aperture, spread itself in all directions, while light was known to be propagated in straight lines? If light consisted in pressure, or motion propagated either in an instant or in time,' urged Newton, it would bend into the shadow. For pressure or motion cannot be propagated in a fluid in right lines beyond an obstacle which stops part of the motion, but will bend and spread every way into the quiescent medium which lies beyond the obstacle.' This was always an insurmountable stumbling block to the theory, until the final establishment by Young of the principle of interference led to a satisfactory explanation founded on the extreme minuteness of the waves of light. But while Newton rejected the doctrine of undulations as opposed to experiment and demonstration, he was almost as cautious in his qualified acceptance of the emission theory. What he did most clearly appreciate was, that either hypothesis, whether physically true or false, might be used as a vehicle for generalising known facts, and so as a step to the settlement of the true theory. It is in this qualified sense that Newton at one time seems to incline to the one theory, and at another to accept the contrary one, though as a matter of physical fact he never absolutely asserted anything as to the constitution of light beyond the experimental fact that it was something which proceeded in straight lines. In the explanation of the colours of thin plates, such as are seen in soap-bubbles, Hooke and Newton were equally successful in propounding theories which accorded with observation; but while Hooke's view was based on the principle of interference, which had been first intimated by Grimaldi and has since revolutionised the theory of light, Newton had recourse to a far less elegant hypothesis, which

served the same purpose of embodying the phenomena. But his original discovery of the different refrangibility of different kinds of light enabled him to demonstrate, with a precision which had not been approached, the true principle of the colours of thin plates, so far as it was independent of any special theory as to the cause of light. The cardinal fact which he established, not merely generally but with arithmetical exactness, was that the bands or rings formed by thin plates of air or water, or any other transparent medium, depended for their magnitude upon the refrangibility of the light. Each colour of the prismatic spectrum was made to fall by turns on the film under examination, and a new confirmation of his first discovery was afforded by the varying diameters of the successive rings. 'It was very pleasant,' he says in describing this experiment, to see the circles swell or contract according as the colour of the light was changed.' The peculiar succession of colours observed, when common light was experimented on, was accounted for at once by the super-position of the unequal bands of the different rays; and in the accurate observation of this class of phenomena, Newton supplied the materials for one of the most striking evidences of the undulatory hypothesis to which, on independent grounds, he had been unable to give his assent. Thus, even in a branch of the subject where he had been clearly anticipated by others, Newton's clearness of perception and aptitude in experiment enabled him to contribute nearly as much to the elucidation of the nature of light as if he had himself invented or adopted the hypothesis which is now almost universally accepted, and which, whether true or false, is at any rate the most comprehensive expression of the multitude of diverse facts which make up the data for a theory of light. Newton's hypothesis was more at fault when he came to discuss the phenomena, then termed the inflection and deflection of light, and now better known under the general designation of the phenomena of diffraction. He himself regarded his experiments as incomplete, and it is possible that, if he had prosecuted them further, he might have anticipated the explanation which was afterwards afforded by the theory of interference. He was not even familiar with the full extent of the observations which had been made by Grimaldi; and when he published his Optics, in 1704, he expressly described this part of the work as an unfinished essay, and concluded it with a series of suggestive queries for future consideration in place of the propositions which he affirmed and proved in those parts of the subject which he had fully matured. Ingenious but immature speculations on the cause of the local colour of terrestrial objects, and on some other isolated optical questions, complete the contributions of Newton to

this department of natural philosophy; and although the same power which built up the theory of the heavens may be traced in many of these optical investigations, it will generally be conceded, that the subject in which Newton displayed the most striking originality of discovery is precisely that in which, apart from his leading discovery, he was least in advance of his contemporaries. Besides the three great subjects of Newton's labours-the fluxional calculus, physical astronomy, and optics-a very large portion of his time, while resident in his college, was devoted to researches of which scarcely a trace remains. Alchemy, which had fascinated so many eager and ambitious minds, and had indirectly contributed to the creation of chemical science, seems to have tempted Newton with an overwhelming force. What theories he formed, what experiments he tried in that laboratory where it is said the fire was scarcely extinguished for weeks together, will never be known. It is certain that no success attended his labours; and Newton was not a man-like Keplerto detail to the world all the hopes and disappointments, all the crude and mystical fancies which mixed themselves up with his career of philosophy. An occasional reference to the transmutation of metals is found in his correspondence; but even his assistant and amanuensis was without the slightest knowledge of the nature and purpose of his experiments. He would sit up till two or three in the morning, sometimes till five or six,' writes Humphrey Newton, evidently a little shocked at his master's proceedings, especially at spring and fall of the leaf; at which times he used to employ about six weeks in his laboratory, the fire scarcely going out either night or day; he sitting up one night and I another, till he had finished his chemical experiments, in the performances of which he was the most accurate, strict, and exact. What his aim might be, I was not able to penetrate into; but his pains, his diligence, at these set times, made me think he aimed at something beyond the reach of human art and industry.' If there were any doubt of the real nature of Newton's work in his laboratory, the remarkable letter which he wrote to a young friend about to travel on the Continent would remove it. With unwonted earnestness he urges him to note any transmutations out of one species into another; as, for example, out of iron into copper, out of one salt into another, and the like, such transmutations being the most luciferous, and many times luciferous experiments in philosophy.' He comes still closer to the subject in a recommendation to inquire about a certain alchemist-a refugee in Holland, who usually went clothed in green-who had been imprisoned by the Pope to extort secrets of great value, and is anxious to ascertain whether his ingenuity be

6

any

any profit to the Dutch.' Many years later we find Newton in correspondence with Locke, with reference to a mysterious red earth by which Boyle, who was then recently dead, had asserted that he could effect the grand desideratum of multiplying gold. By this time, however, Newton's faith had become somewhat shaken by the unsatisfactory communications which he had himself received from Boyle on the subject of the golden recipe; though he did not abandon the idea of giving the experiment a further trial as soon as the weather should become suitable for furnace experiments. It is possible that Vigani, the first Cambridge professor of chemistry, with whom Newton was very intimate, may have shared some of his golden dreams; but beyond the scattered intimations which prove Newton's eager pursuit of the science of alchemy, scarcely any trace of his laboratory experiments is now to be found. Some boastful moderns may be inclined to indulge in a smile at Newton's expense, and regard his belief in the possibility of the philosopher's stone as an illustration of the occasional weakness and credulity of great minds. Certainly, in the present state of chemical science, no one would recommend the pursuit of alchemy as a promising enterprise; but although strong negative evidence exists to discountenance the idea of the essential identity of different metals, the hypothesis is not, and perhaps never will be, absolutely disproved. Quite independently of any auriferous results, Newton was right in regarding transmutations (as all chemical decompositions were then termed) as many times the most luciferous experiments in chemistry, if not in all philosophy; and modern discoveries of the allotropic forms of various substances have established the leading idea of the alchemists, that substances absolutely identical in chemical composition may present themselves under aspects quite as different as those of copper and gold. Perhaps the most that can be said against the students of alchemy, is that they were led by a thirst for wealth to prosecute inquiries to which no known facts gave any semblance of encouragement. Philosophy arrives at truth by following out the hints which nature gives. Alchemists endeavoured to prove a hypothesis which was suggested by nothing but their own love of gain. The wish was father to the thought of the transmutability of metals, and it is not surprising that experiments founded on such a basis should have proved barren of results. Some knowledge of chemistry was incidentally acquired in the prosecution of golden secrets; but Chemistry only began to flourish as a science when she looked for suggestions to the observed processes of nature rather than to the morbid greed of her own adepts.

That these fruitless researches did not altogether displace the

nobler

nobler objects of Newton's ambition, is proved by the circumstance that they were carried on with the greatest vigour during the period of his residence at Cambridge, where he matured his chief discoveries. Still everything that is known of Newton shows him to have been essentially a man of one pursuit at one time; and during the seasons of his furnace work it is not probable that he devoted much thought to investigations which were to bring him a reward richer than gold. The habit of continuous application to one subject was one of the great secrets of Newton's strength, and at every turn we come upon some evidence of his reluctance to be diverted from his immediate occupation to discuss scientific questions which for the time he had laid aside. It was when thus distracted by inopportune controversies from the thoughts with which he was absorbed, that Newton exhibited the occasional petulance which contrasted so strongly with his natural disposition; and it is not an unnatural supposition that the tantalising search for the secret of transmutation may have sometimes been the occasion which led him to reject the intrusion of other subjects. However this may be, the complete obscurity in which the greater part of his chemical experiments were veiled, illustrates an element in Newton's character which had a very sinister influence upon his relations with contemporary philosophers.

The reserve which Newton maintained with reference to his investigations has often been ascribed exclusively to the modesty of his disposition, and Sir David Brewster strongly inclines to this explanation. That Newton-like most men of surpassing eminence was endowed with the true modesty which excludes personal conceit, and rejects inflated notions of the attitude which becomes inquirers into the secrets of nature, is manifest from the whole history of his life. The celebrated saying of his old age, in which he likened himself to a child picking up now and then a prettier shell, or a brighter pebble than ordinary, on the shore of the ocean of truth, was the expression of a sentiment which may be traced in many passages of his life. But the kind of modesty which thus expresses itself is very different from the false modesty which leads a man to misprize the work of his own genius. Of this we find no trace in the career of Sir Isaac Newton; and we believe that, in ascribing to this cause his strange and unfortunate reluctance to give his discoveries to the world, partial biographers have charged Newton with a weakness which was no part of his character. Probably no man of genius ever undervalued his own achievements by comparison with those of other men; though many discoverers, strong in the consciousness of their own powers, may have rated their past work low in comparison