spherical figure of the glasses, and has pointed out several ingenious methods of diminishing them by proper constructions of the eye-pieces. He first showed the advantages of two eyeglasses on the astronomical telescope and double microscope, and gave rules for their construction, which both enlarges the field and shortens the instrument.

Des Cartes published a large work on optics in 1637, and from that period a considerable interval took place, during which optics, and indeed science in general, made but little progress, till the Optica Promota of James Gregory, in 1663, seemed to put them again in motion. The author of this work, a profound and inventive geometer, had applied diligently to the study of optics, and the improvement of optical instruments. The Optica Promota embraced several new enquiries concerning the illumination and distinctness of the images formed in the foci of lenses, and contained an account of the reflecting telescope, still known by the name of its author. The consideration which suggested this instrument was the imperfection of the images formed by spherical lenses, in consequence of which they are not in plane, but in curved surfaces. The desire of removing this imperfection led Gregory to substitute reflection for refraction in the construction of telescopes; and by this means, while he was seeking to remedy a small evil, he provided the means of avoiding a much greater one, with which he was yet unacquainted, viz. that which arises from the unequal refrangibility of light. The attention of Newton was about the same time drawn to the same subject, but with a perfect knowledge of the defect which he wanted to remove. Gregory thought it necessary that the specula should be of a parabolic figure; and the execution proved so difficult that the instrument, during his life, was never brought to any perfection. The specula were afterwards constructed of the ordinary spherical form; and the Gregorian telescope, till the time of Herschel, was more in use than the Newtonian.

Newton, in some of his earliest experimental investigations, applied himself to the improvement of the telescope; but, imagining that Gregory's speculum was neither very necessary nor likely to be executed, he began with prosecuting the views of Des Cartes, who aimed at making a more perfect image of an object by grinding lenses, not to the figure of a sphere, but to that of one of the conic sections. Whilst he was thus employed, three years after Gregory's publication, he had his attention particularly directed to the colours formed by a prism, and, having by means of that simple instrument discovered the different refrangibility of the rays of light, he then perceived that the errors of telescopes arising from that cause alone were some hundred times greater than such as were occasioned by the spherical figure of the lenses. This circumstance forced, as it were, Newton to fall into Gregory's track, and to turn his thoughts to reflectors." The different refrangibility of the rays of light," says he, in a letter to Mr. Oldenburg, secretary of the Royal Society, "made me take reflectors into consideration, and finding them regular, so that the angle of reflection of all sorts of rays was equal to the angle of incidence, I understood that by their mediation optical instruments might be brought to any degree of perfection imaginable, provided a reflecting substance could be found which would polish as finely as glass, and reflect as much light as glass transmits, and the art of communicating to it a parabolic figure be also obtained. Amidst these thoughts I was forced from Cambridge by the intervening plague, and it was more than two years before I proceeded further.”

Optics, as well as all the other branches of natural philosophy, have great obligations to Huyghens. The former was among the first scientific objects which occupied his mind; and his Treatise on Optics, though a posthumous work, is most of it the composition of his early youth. It is written with great perspicuity and precision, and is said to have been a favourite book with Newton himself. Though beginning from the first elements, it contains a full development of the matters of greatest difficulty in the construction of telescopes, particularly in what concerns the indistinctness arising from the imperfect foci into which rays are

united by spherical lenses; and rules are deduced for constructing telescopes which, though of different sizes, shall have the same degree of distinctness, illumination, &c. Huyghens was, besides, a practical optician: he polished lenses and constructed telescopes with his own hands, and some of his object-glasses were of the enormous focal distance of 130 feet.

It is generally supposed that the great reflecting telescopes employed in modern times have a greater focal length than any which had preceded them. This, however, is not the fact, as refracting telescopes of much greater length

had really been attempted to be employed by M. Auzot in France in the seventeenth century. The enormous instruments made by Huyghens and others at the same period were employed without a tube. Huyghens placed his object-glass in a tube at the top of a long pole; the axis of this tube he could command with a silk string, so as to bring it into a line with the axis of another short tube which he held in his hand, and which contained the eye-glass. In the subjoined engraving we have attempted to delineate one of these extraordinary instruments, which for cheapness far surpasses any thing of the kind now used. It will, however, be sufficiently obvious that the important discoveries made with the Herschellian telescope could never have been effected by this instrument.

Prior to the invention of the achromatic glass it was generally supposed that the objects viewed through the telescope must in every form of the instrument be coloured, though some vague hopes, grounded chiefly on the consideration of final causes, were still at times entertained of removing that defect. As the eye consists of two distinct humours, with a horny lens or cornea interposed, it was naturally imagined that such a perfect structure should be imitated in the composition of glasses. This inviting idea is concisely mentioned by David Gregory, the nephew of James, in his little tract on dioptrics. It has also been stated that a country gentleman, Mr. Hall of Chesterhall, in Worcestershire, discovered, about the year 1729, the proper composition of lenses by the united segments of crown and flint-glass, and caused a London artist in 1733 to make a telescope under his directions, which was found on trial to answer extremely well. But, whatever might be the fact, no notice was taken of it at the time, nor indeed till very long after, when circumstances had occurred to call public attention to the subject.

The Newtonian principle was first publicly examined, and a discussion excited which eventually led to a most valuable discovery in optics by a foreign mathematician of great celebrity and transcendant talents. Leonard Euler was a very distinguished mathematician, and especially directed his attention to the subject of optics. Endowed with a penetrating genius and profound capacity, he was capable of pursuing his abstruse investigations with unremitting ardour.and unwearied perseverance. To him the modern analysis stands chiefly indebted for its prodigious extension, and he continued to enrich it in all its departments with innumerable improvements and fine discoveries during the whole course of a most active, laborious, and protracted life. Unfortunately the philosophical character of Euler did not correspond to his superlative eminence as a geometer. Bred in the school of Leibnitz, he had

imbibed the specious but delusive metaphysics of the sufficient reason, and of the necessary and absolute constitution of the laws of nature. He was hence disposed in all cases to prefer the mode of investigating à priori, and never appeared to hold in due estimation the humbler yet only safe road to the knowledge of physical science, by the method of experiment and induction. In the Berlin Memoirs for 1747 he inserted a short paper, in which he deducted from optical principles by a clear analytical process, conducted with his usual skill, the composition of a lens formed after certain proportions with glass and water, which should remove entirely all extraneous colours, whether caused by the unequal refraction of the several rays or by spherical aberration; and, in concluding, he remarked, with high satisfaction, the general conformity of his results with the wonderful structure of the eye.

But this paper met with opposition in a quarter where it could have been least expected. John Dollond, who had afterwards the honour of completing one of the finest and most valuable discoveries in the science of optics, was born in 1706, in Spitalfields, of French parents, whom the edict of Nantes had compelled to take refuge in England from the cruel persecution of an ignorant and bigoted tyrant. Following his father's occupation, that of a weaver, he married at an early age; and being fond of reading he dedicated his leisure moments to the acquisition of knowledge. By dint of solitary application, he made some progress in the learned languages; but he mainly devoted his attention to the study of geometry and algebra, and the more attractive parts of mixed or practical mathematics. He gave instructions in these branches to his son Peter, who, though bred to the hereditary profession, soon quitted that employment, and commenced the business of optician, in which he was afterwards joined by his father. About this time the volume of the Berlin Memoirs, containing Euler's papers, fell into the hands of the elder Dollond, who examined it with care, and repeated the calculations. The result of his examination was communicated by Mr. Short to the Royal Society in 1752, and published in their Transactions for that year. Dollond, as might well be expected, could detect no mistake in the investigation itself, but strenuously contested the principle on which it was built, as differing from the one laid down by Newton, which he held to be irrefragable. "It is, therefore," says he, rather uncourteously, and certainly with little of the prophetic spirit, "it is therefore somewhat strange that any body now-a-days should attempt to do that which so long ago has been demonstrated impossible." The great Euler replied with becoming temper, but persisted in maintaining that his optical principle was a true and necessary law of nature, though he frankly confessed that he had not been able to reduce it yet to practice. Unsatisfied by the arguments of Euler, the humble and unassisted optician applied himself closely to experimental investigation, and, having proved the power of an achromatic arrangement of prisms, next tried so to adapt the opposite refractions as to destroy all extraneous colour. This effect he found to take place when the angle of the wedge had been further increased till the refracting power of the water was to that of the glass in the ratio of five to four. His conclusive experiments were made in 1757, and he lost no time in applying their results to the improvement of the object-glasses of telescopes. Following the proportion just ascertained, he conjoined a very deep convex lens of water with a concave one of glass. In this way he succeeded in removing the colours occasioned by the unequal refraction of light; but the images formed in the foci of the telescopes so constructed still wanted the distinctness which might have been expected. The defect now proceeded, it was evident, from spherical aberration; for, the excess of refraction in the compound lens being very small, the surfaces were necessarily formed to a very deep curvature. But this partial success only stimulated the ingenious artist to make further trials. Having proved that the separation of the extreme rays, or what has been since termed the dispersive power, is not proportioned to the mean refraction in the case of glass and water, he might fairly presume that like discrepancies must exist among other diaphanous substances,

and even among the different kinds of glass itself. Having made this discovery, which at once furnished him with the materials for his achromatic arrangement, Mr. Dollond was encouraged to proceed, with the confident hope of ultimately achieving his purpose. His new researches, however, were postponed for some time by the pressure of business. But, on resuming the enquiry, he found the English crown-glass and the foreign yellow or straw-coloured, commonly called the Venice-glass, to disperse the extreme rays almost alike, while the crystal, or white flint-glass, gave a much greater measure of dispersion. A wedge of crown and another of flint-glass were afterwards ground till they refracted equally, which took place when their angles were respectively 29° and 25°, or the indices of refraction were nearly as 22 to 19; but, on being joined in an inverted position, they produced, without changing the general direction of the pencil, a very different divergence of the compound rays of light. He now reversed the experiment, and formed wedges of crown and flint-glass to such angles as might destroy all irregularity of colour by their opposite dispersions.

In 1758 the labours of Mr. Dollond were rewarded with complete success. "Notwithstanding," says he, in concluding his paper, "so many difficulties as I have enumerated, I have, after numerous trials, and a resolute perseverance, brought the matter at last to such an issue that I can construct refracting telescopes with such apertures and magnifying powers, under limited lengths, as, in the opinion of the best and undeniable judges, who have experienced them, far exceed any thing that has been produced, as representing objects with great distinctness, and in their true colours." The Royal Society voted to Mr. Dollond, for his valuable discovery, the honour of the Copley medal. To this new construction of the telescope, Dr. Bevis gave the name of achromatic, which was soon universally adopted, and is still retained. The inventor took out a patent, but did not live to reap the fruits of his ingenious labours. He died in the year 1761, leaving the prosecution of the business to his son and associate, Peter Dollond, who realized a very large fortune by the exclusive manufacture, for many years, of achromatic glasses, less secured to him by the invidious and disputed provisions of legal monopoly than by superior skill, experience, and sedulous attention. In 1765, the younger Dollond made another and final improvement, to which his father had before advanced some steps. To correct more effectually the spherical aberration, he formed the object-glass of three instead of two lenses, by dividing the convex piece; or he enclosed a concave lens of flint-glass between two convex lenses of crown-glass.

Mr. Tulley has succeeded in applying the achromatic principle of Dollond to the smallest sized glasses, and the improvements effected by Guinaud and Faraday, in the manufacture of glass, have given to this portion of the telescope a degree of excellence which could hardly have been contemplated by the opticians to whose labours we have had occasion to allude. Aided by instruments of this description, astronomers have been able to observe not only stars, planets, and satellites, invisible to the naked eye, but to measure the height of mountains in our own satellite, and discover volcanoes, or burning mountains, on its surface.

We are mainly indebted to the late Sir William Herschel for the improvements that have been made in the reflecting telescope. In 1781, Sir William Herschel began a thirtyfeet aërial reflector; but his mirror, thirty-six inches in diameter, having at one time cracked in the cooling, and at another period run into the fire, from a failure in the furnace, his object was partially abandoned. But, the plan for forming a telescope of extraordinary size having been submitted by Sir Joseph Banks to the king, his majesty offered to defray the expense of it, and, under his patronage, this distinguished optician began, about the end of the year 1785, to construct a telescope of forty feet in focal length. This splendid instrument, which magnified 6450 times, was completed on the 27th of August, 1789; and on the day following Herschel discovered a new satellite belonging to the planet Saturn.

In the article MICROSCOPE we have fully traced the various improvements that have been effected in that instrument from the time of Leuenhoek to the present period; and we may content ourselves with remarking that the microscope, as it is now constructed, amazingly extends the boundaries of the organs of vision, enables us to examine the structure of plants and animals, presents to the eye myriads of beings of whose existence we had before formed no idea, and furnishes the philosopher with an exhaustless field of investigation. "It leads," to use the words of an ingenious writer, "to the discovery of a thousand wonders in the works of his hand who created ourselves, as well as the objects of our admiration; it improves the faculties, exalts the comprehension, and multiplies the inlets to happiness; it is a new source of praise to him to whom all we pay is nothing of what we owe; and, while it pleases the imagination with the unbounded treasures it offers to the view, it tends to make the whole life one continued act of admiration," The oxy-hydrogen microscope is peculiarly fitted to explain, on a large scale, the wonders of the animal and vegetable kingdoms: and its simplicity is such that any person who has a sufficient practical acquaintance with chemistry to produce the steady white light which results from the combustion of the oxy-hydrogen gases on lime may employ the apparatus.

The micrometer is an important part both of the telescopic and microscopic apparatus, and as such must not be passed unnoticed. By the aid of a micrometer we may either measure the size of a planetary body or the smallest object of microscopic investigation. But little use was made of the micrometer for telescopic purposes till the time of Mr. Ramsden, who invented two instruments of this description. Sir William Herschel, also, invented a lamp micrometer, which he employed with one of his powerful telescopes. Cavallo's micrometer is simple and valuable. It consists of a small semi-transparent scale, or slip of mother-of-pearl, about the twentieth part of an inch broad, and of the thickness of common writing-paper. It is divided into a number of equal parts, by means of parallel lines, every fifth and tenth division of which is a little longer than the rest. This micrometer, or divided scale, is situated within the tube, at the focus of the eye-lens of the telescope, where the image of the object is formed, and with its divided edge passing through the centre of the field of view, though this is not absolutely necessary. It is immaterial whether the telescope be a refractor or a reflector, provided the eye-lens be convex, and not concave, as in the Galilean telescope.

The mother-of-pearl micrometer may be applied to a microscope, and it will thus serve to measure the lineal dimensions of an object. In some of the best forms of the micrometer, the eye-tube through which the object is viewed is divided by threads or filaments, and scientific men have often had occasion to regret the difficulty of procuring fibres sufficiently elastic for micrometers. The difficulty of obtaining silver wire of a diameter small enough induced Mr. Troughton to use the spider's web, which he found so fine, opaque, and elastic, as to answer all the purposes of practical astronomy. But, as it is only the stretcher, or long line which supports the web, that possesses these valuable properties, the difficulty of procuring it has compelled many opticians and practical astronomers to employ the raw fibres of unwrought silk, or, what is still worse, the coarse silver wire manufactured in this country. For these Sir David Brewster has succeeded in obtaining a substitute in a delicate fibre, which enables the observer to remove the errors of inflection, while it possesses the requisite property of opacity and elasticity. This fibre is made of glass, which is so exceedingly elastic that it may be drawn to any degree of fineness, and which can always be procured and prepared with facility. This vitreous fibre, when drawn from a hollow tube, will always be of a tubular structure, and its interior diameter may always be regulated by that of the original tube. When such a fibre is formed, and stretched across the diaphragm of the eye-piece of a telescope, it will appear perfectly opaque, with a delicate line of light ARTS & SCIENCES.-VOL. I.

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