firm state of his health; the peevishness of temper which this occasioned; the number of unfinished inventions from which he looked both for fortune and fame; and, above all, his extravagant love of reputation, distracted and broke down the energies of his powerful intellect. In the more extensive inquiries of Newton, he often recognised his own incompleted speculations; and when he saw others reaping that glorious harvest for which he considered he had prepared the ground, and of which he had sown the seed, it was not easy for him to suppress the deep mortification which Newton's success inspired. In the history of science, as in all other history, it is a difficult matter to adjust the rival claims of competitors, when the one was allowed to have completed what the other had begun. He who commences an inquiry and proclaims the results to the world, often goes much further than he has announced, and, pushing his speculations into the very heart of the subject, frequently submits them to the ear of friendship. From the pedestal of his published labours his rival begins his researches, and brings them to a successful issue; while he has, in fact, done nothing more than completed and demonstrated the imperfect speculations of his predecessor. To the world, and to himself, he is in the position of the principal discoverer; but there is still some apology for his rival when he brings forward his unpublished labours; and some excuse for the exercise of personal feeling, when he measures the speed of his rival by his own proximity to the goal.

The conduct of Dr. Hooke would have been viewed with some such feeling, had not his arrogance on other occasions checked the natural current of our sympathy. When Newton presented his reflecting telescope to the Royal Society, Dr. Hooke not only criticised the instrument with undue severity, but announced that he possessed an infallible method of perfecting all kinds of optical instruments, so that "whatever almost

hath been in notion or imagination, or desired in optics, may be performed with great facility and truth.'

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Hooke had been strongly impressed with the belief, that light consisted in the undulations of a highly elastic medium pervading all bodies; and, guided by his experimental investigation of the phenomena of diffraction, he had even announced the great principle of interference, which has performed such an important part in modern science. Regarding himself, therefore, as in possession of the true theory of light, he examined the discoveries of Newton in their relation to his own speculative views, and finding that their author was disposed to consider that element as consisting of material particles, he did not hesitate to reject doctrines which he believed to be incompatible with truth. Dr. Hooke was too accurate an observer, not to admit the general correctness of Newton's observations. He allowed the existence of different refractions, the unchangeableness of the simple colours, and the production of white light by the union of all the colours of the spectrum; but he confidently maintained that the different refractions arose from the splitting and rarefying of ethereal pulses, and that there are only two colours in nature, viz: red and violet, which produce by their mixture all the rest, and which are themselves formed by two sides of a split pulse or undulation.

In reply to these observations, Newton wrote an able letter to Mr. Oldenburg, dated June 11th, 1762, in which he examined with great boldness and force of argument, the various objections of Dr. Hooke, and maintained the truth of his doctrine of colours, as independent of the two hypotheses respecting the origin and production of light. He acknowledged his own partiality to the doctrine of the materiality of light; he pointed out the defects of the undulatory theory; he brought forward new experiments in

confirmation of his former results; and he refuted the opinions of his opponent, respecting the existence of only two simple colours. No reply was made to the powerful arguments of Newton, which our limits prohibit us from giving in full, and Dr. Hooke contented himself with laying before the society, his curious and interesting observations on soap bubbles, and of plates of air, and in pursuing his experiments on the diffraction of light.

Mr. Newton had no sooner silenced this most powerful of his opponents, than he was again called into the field to defend his discoveries against a new enemy. Christian Huygens, an eminent mathematician and natural philosopher, who, like Dr. Hooke, had maintained the undulatory theory of light, transmitted to Mr. Oldenburg various animadversions on the Newtonian doctrine, but though his knowledge of optics was most extensive, yet his objections were nearly as groundless as his less-enlightened countryman. Attached to his own hypothesis respecting the nature of light, namely to the system of undulation, he seems like Hooke, to have regarded the discoveries of Newton, as calculated to overturn it, but his principal objections related to the composition of colours, and particularly of white light, which he alleged could be obtained from the union of two colours, YELLOW and BLUE. To this and similar objections Newton replied, that the colours in question were not simple yellows and blues, but were compound colours, in which, together, all the colours of the spectrum were themselves blended; and though he evinced some strong traces of feeling at being again called upon to answer such frivolous objections, yet his respect for Huygens induced him to enter with patience on a fresh development of his doctrine. Huygens felt the reproof which the tone of this answer so gently conveyed, and in writing to Oldenburg, he made use of the words, that Mr. Newton

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"maintained his doctrine with some concern.' Το this Newton replied, "As for Mr. Huygens' expression, I confess it was a little ungrateful to me, to meet with objections which had been answered before, without having the least reason given me why those were insufficient." But though Huygens appears in this controversy as a rash objector to the Newtonian doctrine, it was afterwards the fate of Newton to play a similar part against the Dutch philosopher, when he published his beautiful law of double refraction in Iceland spar, founded on the finest experimental analysis of the phenomena, though presented as a result of the undulatory system. Newton unhesitatingly rejected it, and substituted for it another law entirely inconsistent with the experiments of Huygens, and with those of all succeeding philosophers.

Even the calm temper of Newton was ruffled on account of these controversies; the satisfaction of humbling all his antagonists was not a sufficient compensation for the disturbance of his tranquillity. In his letter to Oldenburg, containing his first reply to Huygens, he says" I intend to be no farther solicitous about matters of philosophy. And, therefore, I hope you will not take it ill if you find me never doing anything more in that kind; or rather, that you will favour me in my determination, by preventing, so far as you can conveniently, any objections or other philosophical letters that may concern me." In a subsequent letter to the same gentleman, he says-" I had some thoughts of writing a further discourse about colours, to be read at one of your assemblies; but find it yet against the grain to put pen to paper any more on that subject:" and in a letter to Leibnitz, dated December 9th, 1675, he observes-" I was so persecuted with discussions arising from the publication of my theory of light, that I blamed my own imprudence for parting with so substantial a blessing as my quiet to run after a shadow."

CHAPTER III.

Hall invents the Achromatic Telescope. Principles of it explained. Re-invented by Dolland, and Improved by future Artists. Blair's Achromatic Telescope. Mistakes in Newton's Analysis of the Spectrum. Structure of the Spectrum. Colours of thin Plates. Newton determines the law of their production. His Theory of fits of easy reflexion and transmission. Colours of thick Plates. Newton's Theory of the Colours of Natural Bodies explained.

THERE is no fact in the history of science more singular than that Newton should have concluded that all bodies produced spectra of equal length, or separated the red and violet rays to equal distances when the refraction of the mean rays was the same. This opinion, unsupported by experiments, and not even sanctioned by any theoretical views, seems to have been impressed firmly upon his mind with all the force of a self-evident proposition. Even the shortness of the spectrum, observed by his opponent at Liege, did not rouse him to further inquiry; and when, under the influence of this blind conviction, he pronounced the improvement of the refracting telescope to be desperate, he checked for a long time the progress of this branch of science, and furnished to future philosophers a lesson which cannot be too deeply studied.

In 1729, about two years after the death of Sir Isaac Newton, an individual unknown to the scientific world broke the spell in which the subject of the spectrum had been so long and so singularly bound. Mr. Chester More Hall, of More Hall, in Essex, while studying the mechanism of the human eye, was led to suppose that telescopes might be improved by a combination of lenses of different refractive powers, and he actually completed several object glasses upon this