But it may be readily seen that the length is increased four times the focal distance of the eye-glass.

4th. The image of objects might be made to appear upright by employing only two eye-glasses: for this purpose it would be necessary that the first should be at a distance from the focus of the object glass equal to twice its own focal distance; and that the anterior focus of the second should be at twice that distance. Such is the terrestrial telescope with three glasses; but experience has shewn that, by this arrangement, the objects are somewhat deformed, for which reason it is no longer used.

5th. Telescopes with five glasses have also been proposed, in order to bend the rays gradually, as we may say, and to obviate the inconveniences of the too strong refraction, which suddenly takes place at the first eye-glass; and also to increase the field of vision. We have even heard of some telescopes of this kind which were attended with great success; but we do not find that this combination of glasses has been adopted.

6th. Some years ago, a new kind of telescope was invented, under the name of the achromatic, because it is free from those faults occasioned by the different refrangibility of light, which in other telescopes produces colours and indistinctness. The only difference between this and other telescopes is, that the object glass, instead of being formed of one lens, is composed of two or three made of different kinds of glass, which have been found by experience to disperse unequally the different coloured rays of which light is composed. One of these glasses is of crown-glass, and the other of flint glass. An object glass of this kind, constructed according

to certain dimensions determined by geometricians, produces in its focus an image far more distinct than the common ones; on which account much smaller eye-glasses may be employed without affecting the distinctness, as is confirmed by experience. These telescopes are called also Dollond's telescopes, after the name of the English artist who invented them. By the above means, the English opticians construct telescopes of a moderate length, which are equal to others of a far greater size; and small ones, not much longer than opera-glasses, with which the satellites of Jupiter may be seen, are sold under Dollond's name at Paris. M. Antheaume, according to the dimensions given by M. Clairault, made, in that capital, an achromatic telescope of 7 feet focal distance, which when compared with a common one of 30 or 35 feet, was found to produce the same effect.

This invention gives us reason to hope that discoveries will be made in the heavens, which a few years ago would have appeared altogether impossible. It is not improbable even that astronomers will be able to discover in the moon habitations and animals, spots in Saturn and Mercury, and the satellite of Venus, so often seen and so often lost.

To give an accurate idea of the manner in which telescopes magnify the appearance of objects, we shall take, by way of example, that called the astronomical telescope, as being the simplest. If it be recollected that a convex lens produces in its focus an inverted image of objects which are at a very great distance, it will not be difficult to conceive, that the object glass of this telescope will form behind it, at its focal distance, an inverted image of any object towards which it is directed. But, by the construction of the instrument, this image is in

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the anterior focus of the eye-glass, to which the eye is applied; consequently the eye will perceive it distinctly; for it is well known, that when an object is placed in the focus of a lens, or a little on this side of it, it will be seen distinctly through the glass, and in the same direction. The image of the object, which here supplies its place, being then inverted, the eye-glass, through which it is viewed, will not make it appear upright, and consequently the object will be seen inverted.

In regard to the size, it is demonstrated, that the angle under which the image is seen, is to that under which the object is seen, from the same place, as the focal distance of the object glass, is to that of the eye-glass hence the magnified appearance of the object.

In terrestrial telescopes, the two first eye glasses only invert the image; and this telescope therefore must represent objects upright. But having said enough respecting refracting telescopes, we shall now proceed to reflecting ones.

Of Reflecting Telescopes.

Those who are well acquainted with the manner in which objects are represented by common telescopes, will readily conceive that the same effect may be produced by reflection; for a concave mirror, like a lens, paints in its focus an image of distant objects. If means then are found to reflect the image on one side, or backwards, in such a manner as to be made to fall in the focus of a convex glass, and to view it through this glass, we shall have a reflecting telescope. It need therefore excite no surprise that before Newton, and in the time of

Descartes and Mersenne, telescopes on this principle were proposed.

Newton was led to this invention while endeavouring to discover some method of remedying the want of distinctness in the images formed by glasses; a fault which arises from the different re frangibility of the rays of light that are decomposed. Every ray, of whatever colour, being reflected under an angle equal to the angle of incidence, the image is much more distinct, and better terminated in all its parts, as may be easily proved by means of a concave mirror. On this account he was able to apply an object glass much smaller, which would produce a greater magnifying power; and this reasoning was confirmed by experience.

Newton never constructed telescopes of more than fifteen inches in length. According to his method, the mirror was placed in the bottom of the tube, and reflected the image of the object towards its aperture: near this aperture was placed a plane mirror, that is to say, the base of a small isosceles rectangular prism, silvered at the back, and inclined at an angle of 45 degrees. This small mirror reflected the image towards the side of the tube, where there was a hole, into which was fitted a lens of a very short focal distance, to serve as the eyeglass. The object then was viewed from the side, a method, in many cases, exceedingly convenient. Mr. Hadley, a fellow of the Royal Society, constructed, in the year 1723, a telescope of this kind, 5 feet in length, which was found to produce the same effect as the telescope of 123 feet, presented to the Royal Society by Huygens.

The reflecting telescopes, used at present, are constructed in a manner somewhat different. The

concave mirror, at the bottom of the tube, has a round hole in the middle, and towards the other end is a mirror, sometimes plane, turned directly towards the other one, which, receiving the image near the middle of the focal distance, reflects it towards the hole in the other mirror. Against this hole is applied a lens of a short focal distance, which serves as an eye-glass, or for viewing terrestrial objects, in order that they may appear upright; and three eye-glasses are used, arranged in the same manner as in terrestrial telescopes.

A telescope however may be made to magnify much more by the following construction. The large mirror, as in all the others, is placed at the bottom, and has a hole in the centre, before which the eye-glass is applied. At the other end of the tube is another concave mirror, of a less focal distance than the former, and so disposed that the image reflected by the former is painted very near its focus, but at a little farther distance than the focus, from its surface. This produces another image beyond the centre, which is greater as the first one is nearer the focus: this image is formed very near the hole in the centre of the large mirror, opposite to which the eye-glass is in general placed.

This kind of reflecting telescope is called the Gregorian, because proposed by Mr. James Gregory, even before Newton conceived the idea of his; and it is this kind which is at present most in use.

There is also the telescope of Cassegrain, who employs a convex mirror to magnify the image formed by the first concave one. Dr. Smith thought it attended with so many advantages, that he was induced to analyse it in his Treatise on Optics.