made in a frame for this purpose. The axis must be perfectly level, and placed east and west, that the telescope may turn in the plane of the meridian.

In order to observe the instant that a celestial object passes the meridian, there is placed in the telescope a system of wires, generally consisting of five, equi-distant from each other, and perpendicular to the horizon; there is also an horizontal wire bisecting the rest. The middle vertical wire is intended to coincide with the meridian; and the instant that any heavenly body passes this wire is called its transit. The other parallel wires are intended to correct or verify the observation; which is done by taking a mean between the transits over the first wire and the last, or over the second and fourth; or, what is reckoned most correct, a mean of the whole, which is called the reduction of the wires.

There are five adjustments principally necessary to a transit instrument, three of which regard the telescope, and two the axis.

1. The wires should be set perfectly vertical, which is proved by observing that any distant object, cut by a wire, does not vary on moving the instrument up and down, but keeps in the same position over the wire; otherwise, the wires must be turned about till the adjustment is made.

2. The telescope should have no parallax. This is seen by bisecting any distant object with the horizontal wire: and if, on moving the eye up and down a little, the object should appear to separate from the wire, the instrument is said to have a parallax, which must be corrected by placing the object and eye glasses at such a distance asunder, that their foci may meet in the point where the wires are fixed: but when the object glass has been properly fixed by the instrument maker, the observer has only to adjust the eye glass.

3. The telescope should be in collimation.* This is known by observing any object as cut by the meridian wire, and if, on reversing the axis, the object remains cut in the same manner as before, the instrument is in collimation; otherwise, it must be corrected by means of the two small screws in the sides of the telescope; that is, by easing one, and screwing up the other, until the error appears one half diminished; and again, by inverting the axis, and repeating the operation, until the adjustment is properly effected.

4. To level the axis. This is done by means of a screw placed under one of the notches, which raises or depresses that end of the axis at pleasure; and the true horizontal position is proved by a spirit level.

5. To bring the telescope into the meridian. This is performed by an horizontal screw, which moves one end of the axis backward or forward as occasion requires; but to ascertain the meridian with perfect accuracy is a problem of some difficulty as well as importance.

* The line of collimation (i. e. of aiming) is a right line passing from the intersection of the meridian wire with the horizontal, to the centre of the object glass. Or it may be defined a right line passing through the centre of the telescope, and perpendicular to its axis.

WE have made the following extract from Sir I. Newton's works, to shew that he was of opinion that there is an ethereal medium or subtle elastic ether which pervades the universe :

"To proceed to the hypothesis: first, it is to be supposed therein, that there is an ethereal medium, much of the same constitution with air, but far rarer, subtler, and more strongly elastic. It is not to be supposed, that this medium is one uniform matter, but compounded, partly of the main phlegmatic body of either, partly of other various ethereal spirits, much after the manner that air is compounded of the phlegmatic body of air, intermixed with various vapours and exhalations: for the electric and magnetic effluvia, and gravitating principle, seem to argue such variety,

"Is not the heat (of the warm room) conveyed through the vacuum by the vibrations of a much subtiler medium than air?-And is not this medium the same with that medium by which it is refracted and reflected, and by whose vibrations light communicates heat to bodies, and is put into fits of easy reflection and easy transmission? And do not the vibrations of this medium in hot bodies, contribute to the intenseness and duration of their heat? And do not hot bodies communicate their heat to contiguous cold ones, by the vibrations of this medium propagated from them into the cold ones? And is not this medium exceedingly more rare and subtile than the air, and exceedingly more elastic and active? And doth it not really pervade all bodies? And is it not by its elastic force, expanded through all the heavens? May not planets and comets, and all gross bodies, perform their motions in this ethereal medium? And may not its resistance be so small as to be inconsiderable? For instance; if this ether (for so I will call it) should be supposed 700,000 times more elastic than our air, and above 700,000 more rare, its resistance would be about 600,000,000 times less than that of water. And so small a resistance would scarce make any sensible alteration in the motions of the planets in ten thousand years. If any one would ask how a medium can be so rare, let him tell me-how an electric body can by friction emit an exhalation so rare and subtile, and yet so potent? And how the effluvia of a magnet can pass through a plate of glass, without resistance, and yet turn a magnetic needle beyond the glass?"

This is a quotation which has been many times made by those philosophers who favour the undulatory theory of light, to show that Newton was not confident in what is generally said to be his own theory, namely, that light is a real substance.

Achromatic Telescope.*

Achromatic Telescopes differ only from those of the common refracting kind, in the formation and combination of the glasses employed in their construction.

In viewing objects through a refracting telescope, which is not furnished with an achromatic object glass, a kind of aberration is produced, by the different refrangibilities of the various coloured rays of light which form an infinite number of images, neither agreeing perfectly in situation nor in magnitude, so that the objects are rendered indistinct, by an appearance of colours at their edges: this imperfection, however, Mr. Dolland has in great measure obviated, by his achromatic object glasses: the construction of which depends on the important discovery, that some kinds of glass separate the rays of different colours from each other much more than others, while the whole deviation produced in the pencil of light is the same. Mr. Dolland therefore combined a concave lens of flint glass, with a convex lens of crown glass, and sometimes with two such lenses.

The following figure represents an achromatic telescope with a triple object glass, and with Boscovitch's achromatic eye-piece, consisting of two similar lenses, one of which is every way three times as great as the other, their distance being twice the focal length of the smaller.

The concave lens of flint glass was sufficiently powerful to correct the whole dispersion of coloured light produced by the crown glass, but not enough to destroy the effect of its refraction, which was still sufficient to collect the rays of light into a distant focus. For this purpose it is necessary that the focal lengths of the two lenses should be in the same proportion, as the dispersive powers of the respective substances, when the mean deviations of the pencils are equal; that is, in the case of the kinds of glass commonly used nearly in the ratio of seven to ten. Sometimes, also, the chromatic aberration, that is, the error arising from the different refrangibilities of the different rays, is particularly corrected in an eye-piece, by placing a field glass in such a manner, as considerably to contract the dimen

A lens or prism is said to be achromatic, if it form an image free from colour, or if it refracts all the rays of white light to one focus.

A compound lens may be made achromatic, or nearly free from colours, if it consists of two lenses formed of substances of different dispersive powers, the one being convex and the other concave, and having their focal lengths proportional to their dispersing powers.

sions of the image formed by the least refrangible rays, which is nearest to the eye glass, and to cause it to subtend an equal angle to the image formed by the most refrangible rays, this image being little affected by the glass.

The following figure represents one of Mr. Dolland's achromatic telescopes, supported in the centre of gravity, with its rack work motions, and mounted on its mahogany stand,

the three legs of which are made to close up together by means of the brass frame a aa, which is composed of three bars, connected together in the centre piece by three joints, and also to the three legs of the mahogany stand by three other joints, so that the three bars of this frame may lie close against the insides of the legs of the mahogany stand when they are pressed together, for the convenience of car

The brass pin, under the rack work, is made to move round in the brass socket b, and may be tightened by means of the finger screw d, when the telescope is directed nearly to the object intended to be observed. This socket turns on two centres, by which means it may be set perpendicular to the horizon, or to any angle required in respect to the horizon, the angle may be ascertained by the divided arc, and then made fast by the screw e. If this socket be set to the latitude of the place at which the telescope is used, and the plane of this arc be turned on the top of the mahogany stand, so as to be in the plane of the meridian, the socket b being fixed to the inclination of the pole of the earth, the telescope, when turned in this socket, will have an equatorial motion, which is always very convenient in making astronomical observations.

Fig. 2 in the plate, represents a stand to be used on a table, which may be more convenient for many situations, than the large mahogany stand. The telescope, with its rack work, may be applied to either of the two stands, as occasion may require, the sockets on the tops of both being made exactly of the same size. The sliding rods may be applied to the feet of the brass stand, so that the telescope may be used with the same advantages on one as on the other.

The tube A A may be made either of brass or mahogany, of three and half feet long. The achromatic object glass of three and half feet focal distance, has an aperture of two inches and three quarters.

The larger size is with a tube five feet long, and has an achromatic object glass of three inches and one quarter aperture.

The eye tube, as represented by B, contains four eye glasses, to be used for day, or any land objects. There are three eye tubes, as C, which have two glasses in each, to be used for astronomical purposes. These eye tubes all screw into the short brass tube at D. By turning the button or milled head at f, this tube is moved out of the larger, so as to adjust the eye glasses to the proper distance from the object glass, to render the object distinct to any sight, with any of the different eye tubes,

The magnifying power of the three-and-half-feet telescope, with the eye tube for land objects, is forty-five times, and of the five-feet for land objects, sixty-five times. With those for astronomical purposes, with the three-and-half-feet, the magnifying powers are eighty, one hundred and thirty, and one hundred and eighty; and for the fivefeet, one hundred and ten, one hundred and ninety, and two hundred and fifty times.

Stained glasses, as g, are applied to all the different eye tubes, to guard the eye in observing the spots on the sun. These glasses are to be taken off when the eye tubes are used for other purposes.

The rack work is intended to move the telescope in any direction required, and is worked by means of the two handles at h. When the direction of the tube is required to be considerably altered, the worm-screws which act against the arc and the circle must be discharged: then the screw d being loosened, the pin of the rack work will move easily round in the socket b.

For the more readily finding or directing the telescope to any