PIG. I.
(for the principles of the first see TEisscors). With the clock is associated the chronograph as part of a combination for measuring time. Many auxiliary appliances are also brought into use of which the micrometer and the spirit level are the most important. The usefulness of the telescope in measurement does not arise solely from its enabling the observer to see objects otherwise invisible. A telescope with no magnifying power at all would still enable him to determine the directions of the heavenly bodies at any moment with greater accuracy than would otherwise be attainable. The prin ciple on which the telescope is used in celestial measurement will first be explained. Let Fig. 1 represent the section of a telescope •, A B being the object glass, and C the eye-end, where the rays from a star are brought to a focus. The lines converging to the plane E F represent the rays of light from a star reaching the focus. Here they form an image of the star, which the observer sees by looking into the eye-piece at C. The plane, of which the dotted hne E F is a section, passing through the focus at right angles to the telescope is called the focal plane. By changing slightly the di rection in which the telescope is pointed, the rays may come to a focus on any point in this plane not too far from the axis or central line of the telescope. In the focal plane is placed a system of very fine threads which the ob server sees when he looks into the eye-piece. These threads are generally made of fibres of spider-web, a substance so well adapted to this purpose that nothing better has yet come into use. To fix the ideas we shall suppose several cross threads; then the observer by looking into the telescope may see the stars and the cross-threads as represented in Fig. 2. Here we have the images of two stars quite near the crossing point of the threads. The observer moves the telescope until one of the stars is seen exactly at the point of intersection of the two threads. The fundamental principle in the use of the telescope is that when this occurs the star is apparently situated exactly on a straight line passing through the cross-threads, and the centre of the object glass. This line is called the line of sight of the telescope.
Now, let the observer move the telescope until he finds another star, whose image he brings upon the cross-threads. The angle
through which he has moved the telescope from one star to the other, supposing the two stars to be at rest, will then be precisely the angle between the rays of light coming from the two stars. If, then, any system is adopted of de termining through how many degrees, minutes, etc., the telescope has moved, the angular dis tance between the stars is known. The studious reader will remark that, owing to the rotation of the earth, the image of a star seen in a fixed telescope is continually moving across the field of view. To explain the principle we must, however, leave this motion out of account, or suppose it allowed for.
We have next to show how a large angle through which the telescope may be moved is measured. This is done by means of the grad uated circle, a representation of which is shown in Fig. 3. It will be seen that the rim of the circle is divided up into degrees by fine lines as represented in the figure, where, however, only every fifth degree is marked. In the in struments actually used in astronomy, not only is every degree marked, but in the circles for the finest observations the degrees are still further subdivided into spaces of 5', 3', or even 2'. Since there are 360° in a circumference, it follows that in a division to 2' there will be 10,800 of the graduations, or fine marks, on the circle. These marks must all be as nearly equidistant as human art can make them, and the problem of doing this, together with that of making them so fine and sharp that they can be used in the most precise measurement, is one of the most difficult with which the in strument maker has to deal. The way in which the divided circle is used to measure the angular motion of the telescope is shown by the dotted outline of the latter. The circle is attached to it so that both move on an axis concentric with the circle and perpendicular to its plane. Then, when the telescope is turned on this axis, the circle turns with it as a grindstone does on its axis. The distance through which telescope and circle are turned is then measured by means of the graduations. To show how this is done, other appliances must be described.