than one contemporary writer, this useful contri. vance is assigned to à Florentine named Salvini degl' Armati; although he, it is said, would have kept the secret to himself, had it not been for another subject of the same state, Father Alexander de Spina, who, having found it out by the exertion of his own ingenuity and penetration, was too generous to withhold from the world so useful a dis covery. This was about the close of the twelfth century. From this time, magnifying or burning lenses continued to be made of various sizes. But nearly three hundred years more elapsed before any additional discovery of much importance was made in optical science; although in the early part of the sixteenth century Mamolicus of Messina, and, soon after him, Baptista Porta, began once more to direct attention to its principles by their writings and experiments. The latter is said to have first performed the experiment of producing a picture of external objects on the wall of a darkened chamber, by the admission of the light through a lens fixed in a small circular aperture of the windowshutter, the origin of the modern Camera Obscura; and the former made an imperfect attempt to explain the phenomenon of the rainbow. The fortune of ascertaining the true principles of this phenomenon, however, was reserved for Antonio de Dominis, Archbishop of Spalatro, who published his exposition of them in the year 1611.

It appears to have been about this time, also, or not long before, that the telescope was invented; although the accounts that have come down to us regarding this matter are extremely contradictory. As magnifying lenses had been long known and were commonly in use, nothing is more probable than that, as has been suggested, more than one person may, ere this, have accidentally placed two lenses in such a position as to form a sort of rude telescope; and this may account for various evi

dence that has been adduced of something resembling this invention having been in use at an earlier period. But what is certain is, that the discovery of the telescope which made it generally known took place only about the close of the sixteenth century. It seems also to be generally agreed, that it was in the town of Middleburg, in the Netherlands, that the discovery in question was made; and, moreover, that it was made by chance, although the accounts vary as to who was the fortunate author of it. The story commonly told is, that the children of a spectacle-maker, while playing in their father's shop, having got possession of two lenses, happened accidentally to hold them up at the proper distance from each other, and to look through them at the weathercock on the top of the steeple; when, surprised at seeing it apparently so much nearer and larger than usual, they called to their father to come and witness the phenomenon; after observing which he was not long in fabricating the first telescope. The wonderful powers of the new instrument were soon rumoured over Holland and other countries, and the account excited everywhere the greatest interest and curiosity. At last, as we have mentioned in our former volume, it reached Galileo at Venice; and he reinvented the instrument by the application of his own sagacity and scientific skill.

The microscope was also discovered about this time, but by whom is equally uncertain. These instruments, however, contributed greatly to revive a taste for optical investigations; and some of the greatest philosophers of the time, especially Kepler and Des Cartes, successively distinguished themselves in this branch of science, so that some of its most important principles were, ere long, much more accurately ascertained than they had hitherto been, and the phenomenon depending upon them more correctly explained. The early part of the seventeenth century, indeed, exhibits one of the buVOL. I.-N

siest periods in the whole history of optical discovery; nor did the almost constant advance of the science stop till the publication of the Dioptrics of Des Cartes in 1637.

Its next distinguished cultivator was James Gregory, whose Optics appeared in 1663. It was he, as is well known, who first proposed the reflecting telescope, which, on that account, is often called by his name, although he did not succeed in actually constructing such an instrument. This was first accomplished a few years afterward by Sir Isaac Newton, whose investigations on the subject of light, in its whole extent, were destined to create, in regard to that department of physics, nearly as complete a change in the opinions of the age as that which he subsequently effected by the publication of his "Principia," in regard to the mechanism of the heavens. By his celebrated experiment of interposing a prism, or triangular bar of glass in the way of the solar beam, admitted through a small hole into an otherwise darkened chamber, he had made it produce on the wall, not a white circle, as it would have done if allowed to pass on without interruption, but an elongated image or spectrum, as he called it, displaying a series of seven different colours, namely, red, orange, yellow, green, blue, indigo, and violet; hence often spoken of as the seven prismatic colours. This phenomenon proved the hitherto unsuspected facts, first, that white or common light is, in reality, composed of seven different species of rays; and, secondly, that each of these several rays is refrangible in a different degree from the others, that is to say, on passing into a new medium they do not proceed together in one direction, but each starting from the common point of entrance, takes a separate course of its own, so that the beam spreads out into the resemblance of a fan. This is called the divergence, or dispersion of the rays of light; and from

some other experiments which Newton made, he was induced to believe that whatever transparent substances or media refracted a beam of light in the same degree, or, in other words, changed in the same degree its general direction, were also equal in their dispersive powers, or made the different rays separate from one another to the same extent. From this followed a very important consequence. The magnifying powers of the common telescope depended entirely upon the refraction of the light in its passage through the several lenses; but it could not undergo this operation without the rays being at the same time dispersed; and this necessarily threw a certain indistinctness over the image which such telescopes presented to the eye. Here, therefore, was a defect in the refracting telescope which admitted of no cure; for the dispersive bearing the same relation in all substances to the refractive power, you could not obtain the requisite refraction without its inseparable companion, the same amount of dispersion. It was this consideration which made Newton give up all thoughts of improving the refracting telescope, and apply himself, as Gregory had done, to the construction of one which should present its image, not by refracting, but by reflecting, the light from the object.

This rapid sketch of the progress made in the improvement of the telescope up to the beginning of the eighteenth century, will be sufficient to enable the unscientific reader to understand the general nature and importance of a very happy discovery, which since that time has so greatly improved that instrument, and of the author of which, one of the most remarkable examples of self-educated men, we are now about to give some account.

JOHN DOLLOND was born in Spitalfields, London, on the 10th of June, 1706. His parents went to England from Normandy on the revocation of the Edict of Nantes in 1685; and, along with many

thousands more of their countrymen, had established themselves in the above-named district of the English metropolis, in their original business of silkweavers. Dollond's earliest years, also, were spent at the loom; and it had become the more necessary that he should apply himself to his occupation with his utmost industry, in consequence of his father having died while he was yet an infant. Even during his boyhood, however, we are told, he began to show an inclination for the study of the mathematics; and before he was fifteen, he used to amuse himself, during what little leisure he could command, in constructing sundials and solving geometrical problems, although at this time he had scarcely had an opportunity of looking into any book on these subjects. These early habits of study he continued as he grew up towards manhood; so that, notwithstanding the disadvantages under which he laboured, he had, ere long, accumulated a considerable store of learning on his favourite subjects of inquiry. He married early, and an increasing family forced him to make still more unremitted exertions for their support; so that, although he seems now to have become a master manufacturer, he had still less time for private study than ever. But the leisure which business deprived him of during the day, he procured for himself, as many other ardent students have done, by stealing it from the hours usually allotted to sleep. In this manner he continued to improve himself in geometry and algebra; after which he applied himself to different branches of natural philosophy, and with especial ardour, it is recorded, to the kindred departments of astronomy and optics. But Dollond's studies at this time were not confined even to what is commonly called science. He found time to attain a competent knowledge of anatomy, to read a great deal of divinity, and even without any instructer to make himself so far master of the Greek and Latin languages