Cassegrain was a French artist, who proposed this method of construction about the year 1665, and nearly at the same time that Gregory proposed his. It is certain that the length of the telescope is by these means considerably diminished.

The English, for a long time, have enjoyed a superiority in works of this kind. The art of casting and polishing the metallic mirrors, necessary for these instruments, is indeed exceedingly difficult. M. Passement, a celebrated French artist, and the brothers Paris and Gonichon, opticians at Paris, are the first who attempted to vie with them in this branch of manufacture; and both have constructed a great number of reflecting telescopes, some of which are 5 or 6 feet in length. Among the English, no artist distinguished himself more in this respect than Short, though his telescopes were not of great length: besides some of 4, 5 and 6 feet, he made one of 12, which belonged some years ago to the physician of Lord Macclesfield. By applying a lens of the shortest focal distance which it could bear, it magnified about 1200 times. The satellites of Jupiter therefore, seen through this telescope, are said to have had a sensible apparent diameter. But this telescope, as we have heard, is no longer in existence, the large mirror being lost.

The longest of all the reflecting telescopes ever yet constructed, if we except that lately made by Herschel, is one in the king's collection of philosophical and optical instruments at la Meute; it is. the work of dom Noel a Benedictine, the keeper of the collection, and was begun several years before he was placed at the head of that establishment, where he finished it, and where the curious were allowed to see it, and to contemplate with it the

heavens. It is mounted on a kind of moveable pedestal, and, notwithstanding its enormous weight; can be moved in every direction, along with the observer, by a very simple mechanism. But what would be most interesting, is to ascertain the degree of its power, and whether it produces an effect proportioned to its length, or at least considerably greater than the largest and best reflecting telescopes constructed before; for we know that the effects of these instruments, supposing the same excellence in the workmanship, do not increase in proportion to the length.

Huygens' telescope of 123 feet, which he presented to the Royal Society, did not produce an effect quadruple that of a good telescope of 30 feet; and the case must be the same in regard to reflecting telescopes, where the difficulties of the labour are still greater; so that if a telescope of 24 feet produce one half more effect than another of 12, or only the double of one of 6 feet, it ought, in our opinion, to be considered as a good instrument.

We have heard that dom Noel was desirous of making this comparison, and the method he proposed was rational. We have long considered it as the only one proper for comparing such instruments. It is to place at the distance of several hundred feet printed characters of every size, composing bar barous words without any meaning, in order that those who make the experiment may not be assisted by one or two words to guess the rest. The telescope by means of which the smallest characters are read, will undoubtedly be the best. We have seen stuck up, on the dome of the Hospital of Invalids, pieces of paper of this kind, which dom Noel had placed there for the purpose of making

this comparison; but unfortunately such instru ments cannot be brought to one place. Printed characters, such as above described, might therefore be fixed up at a convenient distance from each, without removing the instruments, and persons, appointed for the purpose, ought to go to the different observatories, at times when the weather is exactly similar, and examine what characters can be read by each telescope. By this method a positive answer to the above question would be obtained.

But the largest, and the most powerful, of all the reflecting telescopes, has been lately made by Dr. Herschel, under the auspices of the British monarch; a consequence of which was the discovery of his new primary planet, and of many additional satellites. After a long perseverance in a series of improvements of reflecting telescopes, of the Newtonian form, making them successively larger and more accurate, this gentleman came at length to make one of the amazing size of forty feet in length. This telescope was begun in the year 1785, and completed in 1789. The length of the sheet iron tube is 40 feet, and diameter 4 feet 10 inches. The great mirror is 49 inches in diameter, 3 inches thick, and weighs 2118b. The whole is managed by a large apparatus of machinery, of wheels and pulleys, by means of which it is easily moved in any direction, vertically and sideways. The observer looks in at the outer or object end; from whence proceeds a pipe to a small house near the instrument, for conveying information by sound, backward and forward to an assistant, who thus under cover sets down the time and observations made by the principal observer. The consequences

VOL. II.

of this, and the other powerful machines of this gentleman, have been new discoveries in the heavens of the most important nature.

PROBLEM XLV.

Method of constructing a telescope, by means of which an object may be seen, even when the instrument appears to be directed towards another.

As it is not polite to gaze at any one, a sort of glass has been invented in England, by means of which, when the person who uses it seems to be viewing one object, he is really looking at another. The construction of this instrument is very simple.

Adapt to the end of an opera glass (fig. 40 pl. 11), the object glass of which in this case becomes useless, a tube with a lateral aperture as large as the diameter of the tube will admit, and opposite to this aperture place a small mirror inclined to the axis of the tube at an angle of 45 degrees, and having its reflecting surface turned towards the object glass. It is evident that when this telescope is directed straight forwards, you will see only some of the lateral objects, viz, those situated near the line drawn from the eye in the direction of the axis of the telescope and, reflected by the mirror. These objects will appear upright, but transposed from right to left. To conceal the artifice better, the fore part of the telescope may be furnished with a plane glass, which will have the appearance of an object glass placed in the usual manner.

This instrument, which is not very common in France, is exceedingly convenient for gratifying one's curiosity in the playhouse, and other places of

public amusement, especially if the mirror be so fixed, as to be susceptible of being more or less inclined; for those who use it, while they seem to look at the stage and the performers, may without affectation, and without violating the rules of politeness, examine an interesting figure in the boxes.

We must however observe that the first idea of this instrument is not very new; for the celebrated Hevelius, who it seems was afraid of being shot, proposed many years ago his polemoscope, or telescope for viewing under cover, and without danger, warlike operations, and those in particular which take place during the time of a siege. It consisted of a tube bent in such a manner as to form two elbows, in each of which was a plane mirror inclined at an angle of 45 degrees. The first part of the tube was made to rest on the parapet towards the enemy; the image reflected by the first inclined mirror passed through the tube in a perpendicular direction, and meeting with the second mirror was reflected horizontally' towards the eye glass, where the eye was applied: by these means a person behind a strong parapet could see what the enemy were doing without the walls. The chief thing to be apprehended in regard to this instrument was, that the object glass might be broken by a ball; but this was certainly a trifling misfortune, and not very likely to happen.

Of Microscopes.

What the telescope has performed in the philosophy of the heavenly bodies, the microscope has done in regard to that of the terrestrial: for by the assistance of the latter we have been able to discover'