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Gregory had invented his instrument some years before, and a plan of a similar kind had been suggested by Eskinard as early as 1615. The principal parts of the treatise on optics had been communicated at different times to the Royal Society; besides the experiments on refraction, and the theory of the rainbow, they consist of an elegant analysis of the colors of thin transparent substances, in which the phenomena are reduced to their simplest forms, and of a collection of miscellaneous experiments on the colors produced in cases of inflection or diffraction.

13. With respect to the nature of light, the theory which Newton adopted was materially different from the opinions of most of his predecessors. He considered indeed the operation of an ethereal medium as absolutely necessary to the production of the most remarkable effects of light, but he denied that the motions of such a medium actually constituted light; he asserted, on the contrary, that the essence of light consisted in the projection of minute particles of matter from the luminous body, and maintained that this projection was only accompanied by the vibration of a medium as an accidental circumstance, which was also renewed at the surface of every refractive or reflective substance.

14. In the mean time Bartholin had called the attention of naturalists and opticians to the singular properties of the Iceland crystal, and had hastily examined the laws of its unusual refraction. On this subject Huygens had been much more successful; his analysis of the phenomena of the double refraction is a happy combination of accurate experiment with elegant theory; it was published in 1690, making a part of his treatise on light, the fundamental doctrines of which he had communicated to the academy of Paris in 1678. They scarcely differ in their essential parts from those of our countryman Dr. Hooke, but the subject of colors Huygens has left wholly untouched. Roemer had then lately made the discovery of the immense velocity with which light passed through the celestial regions, by observing the apparent irregularities of the eclipses of Jupiter's satellites; and Huygens readily admitted this property into his system; although Hooke, by a singular caprice, professed himself more ready to believe that the propagation of light might be absolutely instantaneous, than that its motion could be successive, and yet so inconceivably rapid. The merits of Huygens in the mathematical theory of optics were no less considerable than in the investigation of the nature of light; his determination of the observations of lenses was the first refinement on the construction of telescopes.

15. In the year 1720 Dr. Bradley had the good fortune to discover both the existence and the cause of the aberration of the fixed stars. He had for some time observed an irregularity in the places of the stars, which he was wholly unable to explain, and the idea of attributing it to a combination of the effect of the earth's motion in its orbit, with the progressive motion of light, occurred to him first as he happened to observe the apparent direction of the wind on board of a boat which was moving in a transverse direction. He also determined with accuracy the magnitude of the atmospherical refraction, which had been

theoretically investigated by Newton and by Taylor, but never before practically ascertained with sufficient precision. The formula, which Bradley appears to have deduced from observation only, agrees precisely with an approximation which was obtained by Simpson from calculation; but it cannot be considered as rigidly accurate.

16. The optics of Bouguer were first published in 1729, and an improved edition appeared thirty years afterwards; the merits of this author in the examination of the properties of a variety of substances, with respect to the transmission and reflection of light in different circumstances, and in the comparison of lights of different kinds, require to be mentioned with the highest commendation. Dr. Porterfield's investigations of the functions of the eye tended greatly to illustrate the economy of this admirable organ, and some valuable remarks of Dr. Jurin on the same subject were soon after published in Dr. Smith's elaborate treatise on optics, which contains all that had been done at that time with respect to the mathematical part of the science.

17. The invention of achromatic telescopes is with justice universally attributed to our countryman Mr. Dollond; but there is reason to believe that he was not absolutely the first author of the improvement. Mr. Hall, a gentleman of Worcestershire, is said to have discovered, about 1729, Sir Isaac Newton's mistake, in supposing that the rays of different colors must of necessity be equally separated by all surfaces which produce an equal mean refraction; and, by combining the different dispersive properties of different kinds of glass, he constructed, in 1733, several compound object glasses, which were calculated not only for avoiding all appearance of color, but also for correcting the imperfect refractions of the spherical surfaces of the separate lenses. He did not, however, make known the particulars of his investigations, and his invention was soon wholly forgotten. It was in consequence of a discussion with Euler, Klingenstierna, and some other mathematicians, that Mr. Dollond was led to make experiments on the refraction of different kinds of glass; these gentlemen had not questioned the general truth of Newton's opinion respecting the dispersion of the different colors, but Euler had asserted that the eye itself produced a refraction free from the appearance of color, and Klingenstierna had shown the possibility of producing a deviation by refraction, without a separation of color, according to the laws of refraction laid down by Newton himself. When Dollond had once discovered the material distance which exists between the dispersive properties of flint glass and of crown glass, it was easy to produce the combination required; but this ingenious artist was not satisfied with the advantage of freedom from colors only; he adjusted the forms and apertures of his lenses in the most skilful manner to the correction of aberrations of various kinds, and he was also particularly fortunate in being able to obtain, about the time of his discovery, a glass of a quality superior to any that has been since manufactured.

18. The opinion of Euler respecting the eye was, however, by no means well founded; for the

eye acts very differently in rays of different colors, as we may easily observe by viewing a minute object in different parts of a beam of light, transmitted through a prism. It must be allowed that this great mathematician was less fortunate in his optical theories than in many other departments of science; his mathematical investigations of the effects of lenses are much more intricate and prolix than the subject actually requires, and, with respect to the nature and propagation of light, he adopted several paradoxical opinions. Assuming the theory of Huygens, with the additional hypothesis respecting the nature of colors which had been suggested by Newton, and maintained by Pardies and Malebranche; that is, that the difference of colors, like that of tones in music, depends on the frequent differences of the vibrations constituting light; he imagined that opaque bodies are not seen by reflected light, but that their particles are agitated by the impulse of the light which falls on them, and that the vibrations of these particles render the bodies again visible in every direction; he also conceived that the undulations of light are simply propagated through the solid substances of transparent mediums, in the same manner as sound travels through the air. But, on these suppositions, all bodies would have the properties of solar phosphori, and the refraction of the rarest of natural bodies would be incomparably greater than that of the densest is actually found to be and on the whole, although the character of Euler has been so highly and so deservedly respected as to attach a certain degree of authority to all his opinions, so that in this instance the name of Huygens has been almost superseded by that of Euler, yet in fact he has added no augmentative evidence whatever to the theory, but, by inaccurate and injudicious reasoning, has done a real injury to the cause which he endeavoured to support.

19. The researches of Lambert may be considered as a continuation of those of Bouguer; they present us with many interesting observations on the natural history of light, and the properties of various bodies with regard to it. Mr. Lambert first ascertained that a luminous surface emits its light very nearly with equal intensity in all directions, so that any part of it appears almost equally brilliant to an eye placed in any direction, while the light thrown by each square inch or square foot of the surface in any direction differs according to the obliquity of that direction. The mathematical theory of optics is considerably indebted to the labors of Clairault, D'Alembert, and Boscovich; Jeaurat, Beguelin, Redern, and Klugel, have also continued the investigation; their calculations may be of considerable utility to the practical optician; but it requires the ingenuity of a Dollond or a Ramsden to apply the whole of the results to any useful purposes.

20. The experiments of Mazcas on the colors of their plates are mere repetitions of those of Newton, under disadvantageous circumstances. Mr. Dutour has, however, considerably diversified and extended these experiments, as well as those on the colors which are produced in diffracted light, yet without obtaining any general

results of importance. Corparetti's experiments on inflection have every appearance of accuracy; but they are much too intricate to be easily compared with each other, or with those of former observers.

21. The late Dr. Priestley rendered an essential service to the science of optics, considered as a subject for the amusement of the general reader, by an elegant and well-written account of the principal experiments and theories, which had been published before the year 1770. But this work is very deficient in mathematical accuracy; and the author was not sufficiently master of the science to distinguish the good from the indifferent.

22. Mr. Delaval's experiments on colors appear to show very satisfactorily, that all the coloring substances in common use owe their tints to rays, which are separated from white light during its passage through them, and not, as Newton supposed, to the reflection of a particular color from the first surface. It has been observed that Kepler and Zucchius had long ago made experiments nearly similar to those of Mr. Delaval. Dr. Robert Darwin's investigation of the effects of strong lights on the eye appears to comprehend almost all possible varieties of these ocular spectra; but it does not lead to any fundamental analogy capable of explaining the most intricate of them.

23. The phenomena of the unusual atmospheric refraction, which frequently produces double or triple images of objects seen near a heated surface, have been successfully illustrated by Mr. Huddart, Mr. Vince, and Dr. Wollaston; so that at present there appears to be little doubt remaining with respect to their origin. Dr. Wollaston's instrument, for the measurement of refractive densities, very much facilitates the examination of the optical properties of substances of various kinds; he has applied it very successfully to the confirmation of Huygens's theory of double refraction; he has corrected the common opinion respecting the division of the prismatic spectrum; he discovered, without being acquainted with the observations of Ritter, the dark rays which blacken the salts of silver; and he has remarked a single property in some natural as well as artificial crystals, which appear of one color when viewed in the direction of the axis, and of another when in a transverse direction.

24. To Dr. Herschel the sciences of optics and astronomy are equally indebted. He has carried the construction of the reflecting telescope to a degree of perfection far exceeding all that had been before attempted; and the well known improvements which astronomy has derived from his observations are numerous and important. In the course of his researches for the attainment of his more immediate objects he has also had the good fortune to discover the separation of the rays of heat from those of light by refraction; a fact which has been sufficiently established by the experiments of several other persons.

25. We must now furnish a brief sketch of the labors of the experimental philosophers :Euclid, the well known author of the Elements of Geometry, appears to have been the first amongst the ancients celebrated for his know

ledge of optical science. This distinguished geometer did not venture to offer a decided opinion respecting the nature of light; but contented himself with remarking some of its ordinary properties, and applying them to the purposes of human vision. Aristotle has given a definition of light similar to that of motionthe act or energy of a transparent body, inasmuch as it is transparent; and, in proceeding to speculate upon its nature, he altogether denied it to be a substance. This opinion he founded upon the apparently instantaneous transmission of light in any direction. For, as he argued on the general rule, that bodies move with a velocity inversely as the quantity of matter which they contain, light could not, in his estimation, be a material substance, or, the inconceivable velocity of its particles could not be established. As it is not our intention to present an analysis of the opinions of the ancients respecting the nature of light, we shall proceed to a period in its history of the utmost importance to the science of optics; we refer to the foundation of the Cartesian school of philosophy. What has been said respecting the opinions of the ancients is merely intended to show that Descartes, in denying the materiality of light, has only pursued the path marked out by his predecessor Aristotle; though it must be acknowledged that his views of the subject are far more scientific.

26. The opinion of Descartes, and many other modern philosophers of the French school, was, that light is an extremely subtile fluid, pervading the whole sphere of the universe, and receiving from bodies of a luminous nature a series of agitations similar to those excited in a sonorous body by the intervention of the atmospheric air. This theory has undergone some revision, and been somewhat corrected by such modern enquirers as have adopted it; who, to reconcile it with the ordinary and received ideas of the reflection and propagation of light, have supposed its particles to be elastic, and, instead of existing in a state of perfect contiguity, to have small intervals between them. According to this hypothesis, therefore, a ray of light consists of a succession of moleculæ, having a number of oscillations continually repeated. Now, a very great objection to this theory of the French philosopher appears to exist in the rectilinear direction of light; a principle that was discovered by Euclid, and has been subsequently confirmed by minute and conclusive experiments. For, upon this supposition, light would not only be propagated in a direct line, but, like sound, would be transmitted in every direction; the consequence of which would be the exclusion of night, and the impossibility of a phenomenon that has been frequently exhibited in the total eclipse of the

sun.

27. Notwithstanding these great objections to the theory of light we have referred to, so great was the estimation in which the French philosopher was held by the scientific world during the age in which he lived, that no one ventured to dispute its accuracy, until it found an irresistible opponent in Sir Isaac Newton. The theory of this great mathematician and experimentalist, to which modern philosophy has given a decided

preference, consists in the materiality of light. According to this principle, the origin of light is to be traced to an actual emanation, or emission, of particles of luminous bodies thrown off in continued succession, and succeeding one another without interruption. The objections which have been urged against this hypothesis are altogether inconsistent with a just idea of the true nature of light, and possess none of that force by which the objections to the hypothesis of Descartes are accompanied. It has been objected, that, upon the indulgence of this supposition, the rays of light continually emitted from the sun and stars would interfere with each other, and, consequently, prevent their procedure in a rectilinear direction. This difficulty, which is certainly the greatest that has hitherto been adduced, may be readily obviated by taking into consideration the extreme minuteness of the luminous particles, and the disparity which there is between the length of their diameters and relative distance from each other.

28. Having adverted to the Newtonian theory of the materiality of light, it now becomes an object of considerable importance and interest to investigate the nature of its particles. We have already hinted at their extreme minuteness. This may be illustrated by reference to a very simple experiment. If a small hole be made with a needle in a sheet of paper, a spectator lying in a position horizontal to the surface of the earth, and looking through the aperture thus made, would be able to see all the objects in the celestial hemisphere. Now, upon the principle we have stated above, the rays of light, proceeding from the several objects of vision to which the eye of the spectator has been directed, must, of necessity, pass through the aperture at once, without the slightest collision of their particles. Nor will it appear incredible that these myriads of rays should be transmitted through the aperture without the appearance of interruption, when we consider their relative proportion to each other. The utmost efforts of human skill and ingenuity have been directed to the construction of the most delicate optical instruments (as in the case of solar microscopes); and, although they have succeeded in refracting the rays of light to an almost inconceivable extent, yet have hitherto experienced an entire failure in the attempt to discover the distinct particles of this subtile fluid.

29. Another experiment, in illustration of the extreme minuteness of the particles of light, is equally conclusive. If a candle be lighted and placed on an elevation, and no object intervene to obstruct the progress of its rays, it will diffuse itself over an entire space of two miles and upwards in every direction, and form, in itself, a concentric circle, having a diameter of four miles in extent; and this distribution of rays is effected without any sensible diminution of the original luminous body. If the principles we have stated need further confirmation, we may refer to the passage of light through the pores of diamonds, and other gems of a vitreous nature, which have hitherto eluded the detection of the most accurate microscopes.

30. The inconceivable velocity of light is a

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London Published by Thomas Tegg,73,Cheapside December 1.1828.

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