highly illuminated object by means of the button B'; and the magnified image resulting is then thrown on a screen at a distance.

The solar microscope has the great advantage of exhibiting objects to a number of persons at the same time.

In principle, the Oxihydrogen Microscope is the same as the foregoing, only, instead of employing the light of the sun, the rays of a fragment of lime ignited in the flame of an oxihydrogen blow-pipe are used. These rays are converged on the object, and serve to illuminate it. The advantage the instrument has over the solar microscope is, that it can be used at night and on cloudy days.

CHAPTER XLVII.

OF TELESCOPES.

Refracting and Reflecting Telescopes-Galileo's Telescope-The Astronomical Telescope-The Terrestrial-Of Reflecting Telescopes-Herschel's Newton's, the Cassegrairian, Gregory's, Lord Rosse's, Mr. Lassell's, and the Craig Telescope-Determination of their Magnifying Powers-The Achromatic Telescope.

THE telescope is an instrument which, in principle, resembles the microscope, both being to exhibit objects to us under a larger visual angle. The microscope does this for objects near at hand, the telescope for those that are at a distance.

Telescopes are of two kinds, refracting and reflecting. Each consists essentially of two parts-the object-glass or objective, and the eye-piece. In the former, the objective is a lens, in the latter it is a concave mirror.

The distinctness of objects through telescopes is necessarily connected with the brilliancy of the images they give; and this, among other things, depends on the size of the objective.

There are three kinds of refracting telescopes:-1st, Galileo's; 2nd, the Astronomical; 3rd, the Terrestrial.

Fig. 288

H

GALILEO'S TELESCOPE, which is represented in Fig. 288, consists of a convex lens, L N, which is the objective, and a concave eye-glass, E E. Let O B be a distant object, the rays from which are received upon L Ñ, and by it would be brought to a focus, and give the image, MI; but, before they reach this point, they are intercepted by the concave eye-glass, E E, which make them diverge, as represented at H K, and give an erect image, om. This form of telescope has an advantage in the erect position of its image, which is usually presented with great clearness. Its field of view, by reason of the divergence of the rays through the eye-glass, is limited. When made on a small scale, it constitutes the common opera-glass.

THE ASTRONOMICAL TELESCOPE differs from the former in having for its eye-piece a convex lens of short focus compared with that of the object-lens. In this, as in the

S

Fig. 289.

M

former instance, the office of the objective is to give an image, and the eye

piece magnifies it precisely on the same principle that it would magnify any object. In Fig. 289, L N is the objective, and E E the eye-glass; the rays from a distant object, O B, are converged so as to give a focal image, M I. This being viewed through the eye-lens, E E is magnified, and is also inverted. The magnifying power of the telescope is found by dividing the focal length of the objective by that of the eye-lens.

This telescope, of course, inverts, and therefore is not well adapted for terrestrial objects; but for celestial ones it answers very well.

THE TERRESTRIAL TELESCOPE consists of an object-lens, like the foregoing, but in its eye-piece are three lenses of equal focal lengths. The combination is represented in Fig. 290, in which L N is the object-lens, and E E, F F, G G the eyelenses, placed at distances from each other equal to double their focal length. The progress of the rays through the object-lens and

M

E F

E EAC

Fig. 290.

the first eye-glass to X is the same as in the astronomical telescope; but, after crossing at X, they are received on the second eye-lens, which gives an erect image of them at i m, which is viewed, therefore, in the erect position by the last eye-lens, G G.

As the distance at which the image forms from the object-lens is dependent on the actual distance of the object itself, one which is near giving its image further off than one which is distant, it is necessary to have the means of adjusting the eye-piece, so as to bring it to the proper distance from the image, M I. The object-lens is therefore put in a tube longer than its own focus, and in this a smaller tube, bearing the three eye-lenses, immovably fixed, slides backward and forward; this tube is drawn out, until distinct vision of the object is attained.

REFLECTING TELESCOPES are of several different kinds. received names from their inventors.

They have HERCHEL'S TELESCOPE consists of a metallic concave mirror, set in a tube in a position inclined to the axis. It of course gives an inverted image of the object at its focus, and the inclination is so managed as to have the image form at the side of the tube. There it is viewed by an eye-lens, which shows it magnified and inverted. The back of the observer is turned to the object, and the inclination of the mirror is for the purpose of avoiding obstruction of the light by the head. NEWTON'S TELESCOPE consists of a concave mirror, A R, Fig. 291, with

E

F

Fig. 291.

its axis parallel to that of the tube, D E, FG, in which it is

set.

The rays reflected from it are intercepted by a plane mirror, CK, placed at an angle of 45°, on a sliding support, m. They are, therefore, reflected

towards the side of the tube, the image, im, forming at I M, an eye-glass at L magnifies it.

[THE CASSEGRAIRIAN TELESCOPE.-The great speculum of this instrument is perforated like the Gregorian; but the rays converging from the surface

of the mirror, A B, Fig. 292, towards the focus a, are intercepted before they reach that point by a small convex mirror, d, not sufficiently convex to make the rays divergent,

but of such a curvature as to prevent them from coming to a focus till they are thrown back to b, near the aperture in A B, where they form an inverted image, which is viewed by the eye-piece E. This construction has the

Fig. 292.

advantage of requiring a shorter tube than the Gregorian; but the inversion of the image is not corrected; and for this reason probably it has not been much used.

[The small mirror, d, is adjusted by means of a rod turning on a shoulder near the eye-end of the tube (the same as in the Gregorian telescope), and connected by a screw with the apparatus which carries the wire to which the mirror is attained.-Brande's " Dict. of Science, Literature and Art," p. 1220.]

THE GREGORIAN TELESCOPE has a concave mirror, A R, Fig. 293, with an aperture, L, in its centre. The rays from a distant object, O B, give, as before, an inverted image, M I. They are then received on a small concave mirror, K C, placed fronting the great one. This gives an erect image, which is magnified by the eye-lens, P.

R

Fig. 293.

The magnifying power of any of these instruments may be roughly estimated by looking at an object through them with one eye, and directly at it with the other, and comparing the relative magnitude of the two images. In Herchel's telescope the back of the observer is toward the object, in Newton's his side, but in Gregory's he looks directly at it. The latter is, therefore, by far the most agreeable instrument to use. The largest telescopes hitherto constructed are upon the plan of Herschel and Newton.

[Lord ROSSE'S TELESCOPE, which is nearly 50 feet long, with its huge wooden tube Mr. LASSELL'S, with its sheet-iron tube, and the CRAIG TELESCOPE, are all wonders of modern science. Our readers have already become familiar with the last instrument, as we gave a description and illustration of it in a previous section.]

When Sir Isaac Newton discovered the compound nature of light, by prismatic analysis, he came to the conclusion that the refracting telescope could never be a perfect instrument, because it appeared impossible to form an image by a convex lens, without its being coloured on the edges by the dispersion of light. He therefore turned his attention to the reflecting telescope, and invented the one which bears his name. He even manufactured one with his own hands. It is still preserved in the cabinet of the Royal Society of London.

But after it was discovered that refraction without dispersion can be effected, and that lenses can be made to form colourless images in their foci, the principle was at once applied to the telescope; and hence originated that most valuable astronomical instrument, the Achromatic Telescope.

In this the object-glass is of course compound, consisting, as represented

in Fig. 294, of one crown and one flint-glass lens; or as represented in Fig. 295, of one flint and two crown-glass lenses. The principle of its action has been described in Chapter XL. The great

expense of these instruments arises chiefly from the costliness of the flint glass, for it has hitherto been found difficult to obtain it in masses of large size, perfectly free from veins or other imperfections. Nevertheless there are instruments which have been constructed in Germany, with an aperture of thirteen inches. Some of these are mounted on a frame, connected with

Fig. 294.

Fig. 295.

a clock movement; so that when the telescope is turned to a star it is steadily kept in the centre of the field of view, notwithstanding the motion of the earth on her axis. Several large instruments of this description are now in the various Observatories.

SECTION IX.-THE PROPERTIES OF HEAT.-THERMOTICS.

CHAPTER XLVIII.

THE PROPERTIES OF HEAT.

Relations of Light and Heat-Mode of Determining the Amount of HeatThe Mercurial Thermometer-Its Fixed Points-Fahrenheit's, Centigrade, Reaumur's Thermometers-The Gas Thermometer-Differential Thermometer-Solid Thermometers-Comparative Expansion of Gases, Liquids, and Solids.

WHATEVER may be the true cause of light, whether it be undulations in an ethereal medium, or particles emitted with great velocity by shining bodies, observation has clearly proved that heat is closely allied to it.

When a body is brought to a very high temperature, and then allowed to cool in a dark place, though it might be white-hot at first, it very soon becomes invisible, losing its light apparently in the same way that it loses its heat. And we shall hereafter find the rays of heat which thus escape from it may be reflected, refracted, inflected, and polarised, just as though they were rays of light.

In its general relations, heat is of the utmost importance in the system of nature. The existence of life, both vegetable and animal, is dependent on it; it determines the dimensions of all objects, regulates the form they assume, and is more or less concerned in every chemical change that takes place.

Every object to which we have access possesses a certain amount of heat, and so long as it remains at common temperatures, may be touched without pain; but if a larger quantity of heat is given to it, it assumes qualities that are wholly new, and if touched it burns.

To determine, therefore, with precision the quantity of heat which is present in a body when it exhibits any particular phenomenon, it is necessary

that we should be furnished with some means of effecting its measurement. Instruments intended for this purpose are called thermometers.

[Much doubt exists respecting the date of the invention of the thermometer, and the originator is unknown; but it is generally considered to have been invented about the beginning of the 17th century. Vast improvements have been made in its construction since the period of its adoption. To the astronomer Röemer is due the merit of having proposed mercury as the thermometer fluid, instead of the linseed-oil used by Newton. About 1724 Fahrenheit considerably improved the instrument; but since then little or no improvement has taken place in it.]

Some are made of
With a few excep-

Of thermometers we have several different kinds. solid substances, others of liquids, and others of gases. tions they all depend on the same principle the expansion which ensues in all bodies as their temperature rises.

Of these the Mercurial Thermometer is the most common, and for the purposes of science the most generally available. It consists of a glass tube, Fig. 296, with a bulb on its lower extremity. The entire bulb and part of the tube are filled with quicksilver, and the rest of the tube, the extremity of which is closed, contains a vacuum. This glass portion is fastened in an appropriate manner upon a scale of ivory or metal, which bears divisions, and the thermometer is said to be at that particular degree against which its quicksilver stands on the scale.

60 If we take the bulb of such an instrument in the hand, the quicksilver immediately begins to rise in the tube, and finally is stationary at some particular degree, generally the 98th in our thermometers. We therefore say the temperature of the hand it 98 degrees.

50

26

The

In effecting a measure of any kind, it is necessary to have a point from which to start and a point to which to go. same is also necessary in making a scale. One of the essential qualities of a thermometer is to enable observers in all parts of world to indicate the same temperature by the same degree. A common system of dividing the scale must therefore be agreed upon, that all thermometers may correspond.

10 If we dip a thermometer in melting ice or snow, the quicksilver sinks to a certain point, and to this point it will always come, no matter when or where the experiment is made. If we dip it in boiling water, it at once rises to another point. Philosophers in all countries have agreed, that these are the best Fig. 296. fixed points to regulate the scale by, and accordingly they are now used in all thermometers. In the Fahrenheit thermometer, which is commonly employed, we mark the point at which the instrument stands, when dipped in melting snow, 32°, and that for boiling water, 212°, and divide the intervening space into 180 parts, each of which is a degree; and these degrees are carried up to the top and down to the bottom of the scale. [As it is often necessary to reduce the scale of one of the various thermometers in general use to the corresponding degree of another instrument, we consider that the following remarks, extracted from "Brande's Dictionary of Science, Literature, and Art," will be useful to many of our readers :

"For the sake of perspicuity, it is convenient to adapt the expressions to