[Illustration: FIG. 40.--A simple form of refracting telescope.] 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. 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.