proceeding from a point of the object to a point called the focus; and the smaller one, nearest the eye, is called the eye-piece or eye-glass, its province being to render the rays proceeding from the image formed at the focus of the object-glass again parallel and fit for producing distinct vision on entering the eye.

6. The principle of the telescope depends upon a property of light called refraction, by which a ray of light is bent on its passage out of one medium into another, for instance, out of air into water, or out of water into air; out of air into glass, or out of glass into air, &c.; and by this other property that the rays always travel in straight lines, and that an object is seen always in the direction in which the rays enter the eye. If, for example, a coin be placed at the bottom of a basin, and we retire to such a distance that it is just hid by the intervening side, then, by pouring in water (a medium denser than that of air), it will again come into view, having apparently risen, showing that the ray, after leaving the surface of the basin, in its progress towards our eye, is bent or refracted downwards, or farther away from the normal or perpendicular to that point of the surface. Now the water is denser than the air (these being the media in this case through which the light is passing), hence, in passing from a denser to a rarer medium, the ray is bent farther from the perpendicular to the surface separating the two media ; and conversely, in passing from a rarer to a denser medium, it will be bent towards the perpendicular or normal. If we call the angle made by the ray with the normal, in passing out of air, the angle of incidence, and the angle formed with the normal, after passing into water, the angle of refraction, we may enunciate this as the law of refraction, that, in the passage from a rarer to a denser medium, the angle of incidence is greater than the angle of refraction. A most important connexion exists between these two angles, known by the name of the Snellian law of refraction, namely, that the sine of the angle of incidence is for the same media in a constant ratio to the sine of the angle of refraction. This proposition, taken in connexion with the properties

of the spherical surfaces of the glasses, or lenses, used in refracting telescopes, constitutes, when developed, the whole of their theory.

7. The surfaces of the lenses employed are segments of different spheres, and it is by the combination of different curvatures of the lenses with different kinds of glass, and by the choice of the distances at which they are placed from each other, that all the improvements in modern telescopes, admitting of the use of very large object-glasses, are due.

The following engraving gives the principal forms of the lenses ordinarily employed :

In which a is called a double convex lens, b, a convexoplane, c, a double concave, d, a plano-concave, and e, a meniscus. Sections through the centres of the lenses are supposed to be represented; and the line A B, which, in the case of lenses having two spherical surfaces, joins the centres of the surfaces, and in the other cases, passes through the centre of the one surface, and is perpendicular to the other, is called the axis of the lens.

8. Let us now see what will be the effect on a beam of light proceeding from an object so distant that all the rays, from every part of it, may be considered parallel, after its passage respectively through a convex and a concave lens.

Let A B be a double convex lens, C and C' the centres of its spherical surfaces, and D E a ray of light parallel to the axis C C', incident upon the first surface of the lens at E. It will be then bent nearer to the normal C' E, into the direction E F, where it will arrive at the second surface. It will now pass out of glass into the air again, and will be bent farther away from the normal C F into the direction F G, where it will cross the axis, the bending being in each case towards the axis. If the lens be thin, and the curvatures

small, every other parallel ray will cross the axis very

D

B

Fig. 2.

near to the point G, which is called the principal focus of the lens, and denotes the image of a point of an object at an infinite distance, lying in the direction C' C produced. Rays coming from other points of the object, not lying in the line C' C produced, would fall on the lens obliquely, and would form an image either above or below G; and thus would an image of the whole object be formed. This image will plainly be inverted, since the rays proceeding from a point of the image above the line C C' will, after crossing the lens, form an image below C C', and vice versâ.

Since, however, each point of the image is formed by rays converging towards that point, and afterwards diverging, they require to be again rendered parallel, and this is effected by means of another glass or lens, alled the eye-piece, placed at a distance from the image equal to its focal length. A telescope thus constructed is called the astronomical telescope, from its being generally used for the purposes of astronomy.

This

9. A telescope made, however, according to the principles now explained, would be a very bad one; and the image produced, instead of being distinct, would be tinged with all the colours of the rainbow, and would present only a confused resemblance to the object. arises from another property of light called dispersion. Every beam of white light is composed of beams of light of the most brilliant colours, and whenever it is refracted, by being made to pass through a medium of different density, these colours are rendered visible.

Thus, if an ordinary prism be held in the sun-light, a spectrum, as it is called, brilliantly coloured with the tints of the rainbow, may be thrown upon the opposite wall. The several colours, as defined by Newton, are red, orange, yellow, green, blue, indigo, and violet,—the red occupying one end of the spectrum, and the violet the other, these being respectively the least and the most refrangible rays.

10. Now, it will readily be seen, that every lens acts as a prism, by refracting the rays of light, and, therefore, the images, formed by means of such lenses, will be coloured. To correct this fault, or to render the telescope achromatic, as it is called, a combination of lenses is employed. It was discovered by Dollond, the celebrated optician, that different kinds of glass have different dispersive powers; so that, by properly combining two single object-glasses, the one made of crown glass, and the other of flint glass, the rays could be made to pass through them, so as to produce a nearly colourless image. Such a compound objectglass is called an achromatic object-glass, and consists of a convex lens of crown glass, and a concave lens of flint glass. In the same way, by means of a combination of lenses, an achromatic eye-piece is produced, for the purpose of transmitting the image colourless to the eye. Of these eye-pieces, two sorts, called the Huygenian or negative, and the positive, and each consisting of a combination of two lenses, are used in connexion with the astronomical telescope. The negative eyepiece is not adapted for the application of the micrometer, or apparatus usually employed for measuring small angles, but is well adapted for viewing the heavenly bodies. But for all ordinary purposes of an observatory, the positive eye-piece is employed.

LESSON II.

MOUNTING TELESCOPES.

"I will fetch my knowledge from afar, and will ascribe righteousness to my Maker."

1. AFTER having obtained a good telescope, the next point to be attended to is the mounting of it. This is a

point of great importance, for, in the use of high powers, it is necessary to be able to direct the telescope accurately in the direction of the object, and then to keep the object steadily in the field of view while under examination. Now, we know that all the objects in the heavens are carried by the diurnal motion in circles parallel to each other, and perpendicular to the polar axis, and, therefore, if we would comfortably observe any object, our telescope must have just such a mounting as will enable us to turn it round so as to follow the object at pleasure. In fact, the telescope produced to the heavens must sweep out a conical surface, of which our eye is the apex or vertex, and whose base is the diurnal circle described by the stars. We should evidently attain this object, if we could fix the telescope by the middle of its tube to an axis placed parallel to the earth's axis, and revolving freely, and if we were also to allow the telescope to revolve freely in the plane of the axis; for, having once found a star in the field of view, we could, by fixing the telescope relatively to the axis, and turning the axis round, follow it in its diurnal course.

2. A telescope with such a mounting is called an equatorial instrument, or a parallactic instrument. Divided circles are attached to it, the one perpendicular to the axis of revolution, or polar axis, called the hour circle; and the other in the plane of the polar axis, or parallel to it, called the declination circle. The hour circle serves to measure the star's distance from the meridian ; and the declination circle serves to measure the distance from the pole or from the equator. By means of the hour angle and the sidereal time observed by the clock, the right ascension of objects can be found, as well as their declination by the reading of the declination circle.

It is still a troublesome operation, when observing a faint or difficult object, to give such a delicate and uniform motion to the telescope as shall cause the stars to remain fixed in the field of view. To obviate this difficulty, clock-work is usually attached to all large equatorials, so regulated as to carry the instrument