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Galilean telescope, which is the same in principle as the ordinary opera glass. In this case the objective A condenses the light from the object observed, and would naturally make a small image at F, but the cone of rays, before reaching the focus, is intercepted by the double concave eye-piece C, and thence conveyed to the eye in erect position. Fig. 6 shows the Porro Prism instrument, the most modern and efficient form of terrestrial telescope. The objective and the lenses are in the same relation to each other as was first illustrated in the astronomical telescope, Fig. 1. Two doublereflecting, 90-degree prisms are inserted within the cone of rays between the eye-piece and the objective (Fig. 6); their mission being to erect the image which, in the ordinary refracting telescope, is shown inverted. This system was the invention of the Italian engineer, Porro, who patented it in France about 1850.

The magnifying power of telescopes is usually expressed in diameters and is indicated by the ratio of the focal length of the objective to that of the combination of lenses forming the eye-piece. For example, the Lick telescope has a focal length of 56 feet or 672 inches. therefore, an eye-piece of one inch focus is used

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power of 100 diameters is four times as brilliantly illuminated as would be the case if an eye-piece giving 200 diameters were used. It follows, therefore, that the observer will use the power best adapted to his purpose, both as to magnification and light.

The telescope giving the minimum power is the opera glass, usually magnifying two and one-half or three diameters, which is sufficient for indoor use, while for outdoor use the Galilean binocular has a power of four or five diameters, and the prism binocular of six, eight, 10 or even 12 diameters. The eightpower is, however, considered as high as can be held in the hands with sufficient steadiness to give the best results. The power used in terrestrial telescopes steadily mounted on a tripod usually ranges from 15 to 100 diameters, depending on the condition of the atmosphere.

The most important element in a refracting telescope is the objective, and, in a reflecting telescope, the mirror or speculum. The objective of the early refracting telescope was a double-convex lens, which could not give a distinct image because it separated each ray of light into its various prismatic colors, and each color, having a refracting power different from

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The 100-foot Dome, Mount Wilson Solar Observatory, Pasadena, Calif,

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