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lished labours his rival begins his researches, and brings them to a successful issue; while he has in reality done nothing more than complete and demonstrate the imperfect speculations of his predecessor. To the world, and to himself, he is no doubt in the position of the principal discoverer: but there is still some apology for his rival when he brings forward his unpublished labours; and some excuse for the exercise of personal feeling, when he.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 and imagination, or desired in optics, may be performed with great facility and truth.”

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 scruple 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 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 the two sides of a split pulse or undulation.

In reply to these observations, Newton wrote an able letter to Oldenburg, dated June 11, 1672, in which he examined with great boldness and force of argument the various objections of his opponent, 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 opini yns of Hooke respecting the existence of only two simple colours. No reply was made to the powe ul arguments of Newton; and Hooke contented hi nself with laying before the Society his curious obs rvations on the colours of soap-bubbles, and of plates of air, and in pursuing his experiments on the diffraction of light, which, after an interval of two years, he laid before the same body.

After he had thus silenced the most powerful of his adversaries, Newton was again called upon to defend himself against a new enemy. Christian Huygens, an eminent mathematician and natural philosopher, who, like Hooke, had maintained the undulatory theory of light, transmitted to Oldenburg various animadversions on the Newtonian doctrine ; but though his knowledge of optics was of the most extensive kind, yet his objections were nearly as groundless as those of his less enlightened

countryman. Attached to his own hypothesis re. specting the nature of light, namely, to the system of undulation, he seems, like Dr. 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 put upon his defence, yet his high 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 used the expression, that Mr. Newton “maintained his doctrine with some concern." To this our author replied, “ As for Mr. Huygens's 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 answers were insufficient.” But though Huygens appears in this controversy as a rash objector to the Newtonian doctrine, it was afterward the fate of Newton to play a similar part against the Dutch philosopher. When Huygens 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 not only rejected it, but substituted for it another law entirely inconsistent with the experiments of Huygens, which Newton himself had praised, and with those of all succeeding philosophers.

The influence of these controversies on the mind of Newton seems to have been highly exciting.

Even the satisfaction of humbling all his antago. nists he did not feel as a sufficient compensation for the disturbance of his tranquillity. “I intend," says he,* * 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 any thing 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 let. ter in 1675, 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 the 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 V.

Mistake of Newton in supposing that the Improvement of Refracting

Telescopes was hopeless--Mr. Hall invents the Achromatic Telescope -Principles of the Achromatic Telescope explained-It is re-invented by Dollond, and improved by future Artists - Dr. Blair's Aplanatic Telescope-Mistakes in Newton's Analysis of the Spectrum-Modern Discoveries respecting the Structure of the Spectrum.

The new doctrines of the composition of light, and of the different refrangibility of the rays which compose it, having been thus established upon an impregnable basis, it will be interesting to take a general view of the changes which they have under

I atter to Oldenburg in 1672, containing his first reply to Huygens. gone since the time of Newton, and of their influence on the progress of optical discovery.

There is no fact in the history of science more singular than that Newton should have believed 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 upon his mind with all the force of an axiom.* Even the shortness of the spectrum observed by Lucas 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, an individual unknown to science broke the spell in which the subject of the spectrum had been 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 principle. The steps by which he arrived at such a construction have not been recorded; but it is obvious that he must have discovered what escaped the sagacity of Newton, that prisms made of different kinds of

* In an experiment made by Newton, he had occasion to counteract the refraction of a prism of glass by another prism of water; and had he completed the experiment, and studied the result of it, he could not have failed to observe a quantity of uncorrected colour, which would have led him to the discovery of the different dispersive powers of hodies. But in order to increase the refractive power of the water, he mixed with it a little sugar of lead, the high dispersive power of which seems to have rendered the dispersive power of the water equal to that of the glass, and thus to have corrected the uncompensated colour of the glass prism.

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