Fig. 39.—Essential parts of a reflecting telescope.
78. Telescopes.—The simplest kind of telescope consists of a concave mirror to produce images, and a magnifying glass, called an eyepiece, through which to examine them; but for convenience' sake, so that the observer may not stand in his own light, a small mirror is frequently added to this combination, as at H in [Fig. 39], where the lines represent the directions along which the energy is propagated. By reflection from this mirror the focal plane and the images are shifted to F, where they may be examined from one side through the magnifying glass E.
Such a combination of parts is called a reflecting telescope, while one in which the images are produced by a lens or combination of lenses is called a refracting telescope, the adjective having reference to the bending, refraction, produced by the glass upon the direction in which the energy is propagated. The customary arrangement of parts in such a telescope is shown in [Fig. 40], where the part marked O is called the objective and V E (the magnifying glass) is the eyepiece, or ocular, as it is sometimes called.
Fig. 40.—A simple form of refracting telescope.
Most objects with which we have to deal in using a telescope send to it not light of one color only, but a mixture of light of many colors, many different wave lengths, some of which are refracted more than others by the glass of which the lens is composed, and in consequence of these different amounts of refraction a single lens does not furnish a single image of a star, but gives a confused jumble of red and yellow and blue images much inferior in sharpness of outline (definition) to the images made by a good concave mirror. To remedy this defect it is customary to make the objective of two or more pieces of glass of different densities and ground to different shapes as is shown at O in [Fig. 40]. The two pieces of glass thus mounted in one frame constitute a compound lens having its own focal plane, shown at F in the figure, and similarly the lenses composing the eyepiece have a focal plane between the eyepiece and the objective which must also fall at F, and in the use of a telescope the eyepiece must be pushed out or in until its focal plane coincides with that of the objective. This process, which is called focusing, is what is accomplished in the ordinary opera glass by turning a screw placed between the two tubes, and it must be carefully done with every telescope in order to obtain distinct vision.
79. Magnifying power.—The amount by which a given telescope magnifies depends upon the focal length of the objective (or mirror) and the focal length of the eyepiece, and is equal to the ratio of these two quantities. Thus in [Fig. 40] the distance of the objective from the focal plane F is about 16 times as great as the distance of the eyepiece from the same plane, and the magnifying power of this telescope is therefore 16 diameters. A magnifying power of 16 diameters means that the diameter of any object seen in the telescope looks 16 times as large as it appears without the telescope, and is nearly equivalent to saying that the object appears only one sixteenth as far off. Sometimes the magnifying power is assumed to be the number of times that the area of an object seems increased; and since areas are proportional to the squares of lines, the magnifying power of 16 diameters might be called a power of 256. Every large telescope is provided with several eyepieces of different focal lengths, ranging from a quarter of an inch to two and a half inches, which are used to furnish different magnifying powers as may be required for the different kinds of work undertaken with the instrument. Higher powers can be used with large telescopes than with small ones, but it is seldom advantageous to use with any telescope an eyepiece giving a higher power than 60 diameters for each inch of diameter of the objective.
The part played by the eyepiece in determining magnifying power will be readily understood from the following experiment: