countryman. Attached to his own hypothesis respecting 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." 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.

To

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

Even the satisfaction of humbling all his antagonists 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 letter 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 etter 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 rouge 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 bodies. 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,

glass produced different degrees of separation of the red and violet rays, or gave spectra of different lengths when the refraction of the middle ray of the spectrum was the same.

In order to explain how such a property led him to the construction of a telescope without colour, or an achromatic telescope, let us take a lens LL of crown or plate glass, whose focal length LY is about twelve inches. When the sun's rays SL,

Fig. 6.

S'

SL fall upon it, the red will be refracted to R, the yellow to Y, and the violet to V. If we now place behind it a concave lens 7 of the same glass, and of the same focus or curvature, it will be found, both by experiment and by drawing the refracted rays, according to the rules given in elementary works, that the concave glass will refract the rays LR, LR into LS', LS', and the rays LV, LV into LS', LS' free of all colour; but as these rays will be parallel, the two lenses will not have a focus, and consequently cannot form an image so as to be used as the object-glass of a telescope. This is obvious from another consideration; for since the curvatures of the convex and concave lenses are the same, the two put together will be exactly the same as if they were formed out of a single piece of glass, having parallel surfaces like a watch-glass, so that the parallel rays of light SL,

SL will pass on in the same direction LS', LS' affected by equal and opposite refractions as in a piece of plane glass.

Now, since the convex lens LL separated the white light SL, SL into its component coloured rays, LV, LV being the extreme violet, and LR LR the extreme red; it follows that a similar concave lens of the same glass is capable of uniting into white light LS', LS' rays, as much separated as LV, LR are. Consequently, if we take a concave lens ll of the same, or of a greater refractive power than the convex one, and having the power of uniting rays farther separated than LV, LR are, a less concavity in the lens will be sufficient to unite the rays LV, LK into a white ray LS'; but as the lens ll is now less concave than the lens LL is convex, the concavity will predominate, and the uncoloured rays LS', LS' will no longer be parallel, but will converge to some point O, where they will form a colourless or achromatic image of the sun.

The effect now described may be obtained by making the convex lens LL of crown or of plate glass, and the concave one of flint glass, or that of which wineglasses are made. If the concave lens Il has a greater refractive power than LL, which is always the case, the only effect of it will be to make the rays converge to a focus more remote than O, or to render a less curvature necessary in ll, if O is fixed for the focus of the combined lenses.

Such is the principle of the achromatic telescope as constructed by Mr. Hall. This ingenious individual employed working opticians to grind his lenses, and he furnished them with the radii of the surfaces, which were adjusted to correct the aberration of figure as well as of colour. His invention, therefore, was not an accidental combination of a convex and a concave lens of different kinds of glass, which might have seen made merely for experiment; but it was a complete achromatic tele