who lived eighteen centuries after him, assert, that more than two parhelia are never seen at once; while in reality, by proper attention, a much greater number is frequently observed. For instance, five Suns were seen at Rome on the 29th of march, 1629; and seven at Dantzic, on the 20th of february, 1661; &c. Now is it possible, says Huygens, that in so few years six or seven parhelia have appeared, each consisting of more than two Suns, and that the same phenomenon never occurred before? No doubt formerly the two lateral parhelia, which are in fact the largest, were alone considered as true parhelia, and no attention was paid to the others, as being more faint and languid. Des Cartes undertook to explain all these appearances; but his explanation was somewhat vague, and in certain respects even erroneous. Huygens corrected it, and by an accurate application of the principles of catoptrics and dioptrics, which were become better known, he perfectly accounted for all the circumstances attending parhelia. For paraselenes the theory is the same.

I have already spoken in general terms of the utility of the telescope in astronomy, but this is a proper place to give a more perfect account of it, and likewise to say something of the microscope, another instrument of the same kind, which has rendered no less service to natural history and physics, than the telescope to astronomy.

It is the common opinion, that we owe the first invention of the telescope to James Metius; and it is placed at the beginning of the seventeenth century. Such too is the sentiment of des Cartes, who wrote

in Holland about thirty years after the discovery. On this subject he expresses himself as follows at the beginning of his Dioptrics; and though the passage is somewhat long, perhaps the reader will see it here with pleasure.

The whole conduct of our life depends on our senses, among which that of sight being the most noble, and the most universal in it's application, it is unquestionable, that those inventions, which increase it's power, are of all the most useful. And it is not easy to find one, that shall increase it more than those wonderful telescopes, which, though their date is so recent, have already discovered new stars in the firmament, and other new objects upon Earth, in greater number than those we had seen before: so that extending our view much farther, than the imagination of our forefathers had been able to reach, they seem to have opened to us a path, by which we may attain a much greater and more perfect knowledge of nature, than they possessed. It is about thirty years since James Metius, of the town of Alcmaer in Holland, a man who had never studied, though he had a father and a brother professors of mathematics, but who took particular delight in making mirrors and burning glasses, forming them. in winter even of ice, as experience has shown may be done, having on this account glasses of various forms, fortunately thought of looking through two; one of which was a little thicker at the centre than at the edges, the other on the contrary much thicker at the edges than in the centre; and he applied them so happily to the two extremities of a tube, that the first

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first of the telescopes of which we speak was composed; and it is wholly after the pattern of this, that all the others we have since seen were made, &c.'

Others relate, that the children of a spectacle maker of Middleburg in Zealand, with whose name we are unacquainted, playing in their father's shop, remarked, that when they put two spectacle glasses one before the other, and looked through them both at the weathercock of a neighbouring steeple, it appeared larger than usual. The father, struck with this singularity, thought of adjusting two glasses on a board, by means of brass rings, which might be brought nearer to each other, or farther off, at pleasure. Thus he was enabled to see better, and at a greater distance; and at length proceeded to place the glasses in a tube, and thus formed a telescope. There are still other opinions on the origin of this instrument, which I shall not recite: but I shall observe, that the testimony of such a man as des Cartes in favour of James Metius ought to have very great weight.

The pretensions of the italians, who have wished to ascribe the invention of telescopes to Galileo, cannot be supported: for Galileo himself says, that, being at Venice when the first rumour of this discovery was spread there, he waited for letters from Paris, to assure himself of the wonders, which Fame reported; and that, after having received a confirmation of them, he sought for the construction of the instrument in the laws of refraction, and thus discovered it. Being in possession of the principle, he by degrees formed a telescope, which magnified the diameter of an object about thirty times; and with

this he discovered the satellites of Jupiter, the spots on the Sun, &c. Thus he merely divined the mechanism of the telescope, from the description that was sent him of it's effects; and this part of the discovery does him sufficient honour, without endeavouring to exaggerate it.

The telescope of Galileo, otherwise called the dutch telescope, is composed of a convex object glass, and a concave eye glass, or planoconcave glass, placed between the object glass and it's focus, in such a manner, that the axes of the two glasses are in the same line, and their foci coincide in the same point. The rays, which the object glass tends to unite, become parallel as they issue from the eye glass, and form at the common focus a perceptible image, which represents the object in it's natural position. The field of telescopes of this sort is very small; and the longer the tube, the smaller the field. This inconvenience has occasioned their disuse in astronomy, where a certain extent of field is required, and at the same time long tubes. They are not now employed therefore, except for short distances.

Some years after the invention of this telescope, Kepler proposed another, which was imperceptibly adopted by all astronomers, and is called the astronomical telescope. This has a convex object glass, and for it's eye glass a lens convex on one or both sides, placed so that it's focus coincides with that of the object glass, and that this common focus falls betwen the two glasses. It shows objects inverted; but it has the advantage of an extensive field and a long tube.

There

There is a third sort of telescope, the perspective glass commonly employed for terrestrial objects, which is nothing but the preceding, with the addition of two glasses for bringing the object to it's natural erect position.

All these telescopes are purely dioptrical, because the simple refraction of light alone is employed in them. There are others more complicated, in which both reflection and refraction are combined, and which for this reason are called catadioptric, or reflecting telescopes. Such are the gregorian telescope, and the newtonian, descriptions of which may be seen at full length in books on optics,

The microscope is an instrument founded on the same principles as the telescope. We know neither the precise time of it's invention, nor the name of the inventor; though it is commonly supposed, that it was Cornelius Drebbel, and that the first appeared about 1618, or 1620. There have been long disputes on this subject, into which I shall not enter. Some writers have greatly depreciated the merit of Drebbel but the truth is, he received an excellent education at Alcmaer, his native place, and was well versed in all the physical knowledge of his time.

There are several sorts of microscopes; but the most simple of all is a glass convex on both sides, and commonly called a convex lens. By placing it so, that it's focus falls on the part we would inspect, the rays, which are parallel as they issue from the lens, form a strong image of the object. Sometimes, instead of a lens, a small sphere of glass is employed, which is easily formed by melting a bit of glass in the