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CHAPTER XII.

Professors of Optical Discovery.-Dollond; Ramsden; Herschel; Thomas Phelps and John Bartlett; Fraunhofer; Palitzch.

THE truth, as we have already remarked, with regard to many of the inventions mentioned by Friar Bacon, probably is, that he had rather deduced them as possibilities from the philosophical principles in which he believed, than actually realised them experimentally. Among others, certain optical instruments to which he attributes very wonderful powers existed merely, there can be little doubt, as conceptions of his mind, and had never been either fashioned or handled by him.

The invention of spectacles, however, may be considered as having been traced, on evidence of unusual clearness in such matters, to about the time of the death of Bacon. By the testimony of more than one contemporary writer this useful contrivance is assigned to a 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 discovery. 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 window-shutter, 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 evidence 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 spectaclemaker, 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 weather-cock 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 re-invented 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 phenomena depending upon them more correctly explained. The early part of the seventeenth century, indeed, exhibits one of the busiest 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 afterwards, by Sir Isaac Newton, whose investigations on the subject of light, * See the Life of Galileo, in the Library of Useful Knowledge,

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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 phenonenon 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, on the 10th of June, 1706. His parents had come to this country 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 silk-weavers. 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 shew an in

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