62 THE UNIVERSE A VAST SYSTEM OF PARTS to the naked eye; a five-inch object glass has about 400 times the area of the pupil of the eye (which is about | inch in diameter) and can collect about 400 times as much light. The largest refractor in the world is the Yerkes telescope at Lake Geneva, Wisconsin. The Yerkes 40-inch telescope will collect about Fig. 4. The 40-inch tck-scoiic al ihe Yerkes Observatory with rising floor at its lowest position. (Photograph from the Yerkes Observatory, reprinted by permission of the Chicago University Press.) edged, blurred appearance to dis- The unaided eye can observe that the tant trees and other objects. 25,600 times as much light as the eye and thus render visible that are stars 25,600 times too distant to be visible to the naked eye. The next largest refracting telescope is the 36-inch Lick telescope at the Lick Observatory on Mount Hamilton near San Jose, California. James Lick bequeathed three million dollars to build the telescope and observatory, which was completed in 1888. The object glass for this telescope was cast in France and polished in America. Just as light is bent as it passes from air through glass, so it is bent by passing from less dense into more dense air (cold or hot). Variations in the moisture content of the air likewise cause bending of the light. Everyone has observed how the air seems to quiver over a hot surface, lending a soft, feathery- stars near the horizon seem to flicker because of the optical aberrations produced by the varying strata of air. To reduce this error as far as possible, telescopes are located, whenever possible, on mountains whose locations have been selected after a great deal of investigation. Another important point to consider in location is to place the telescope where light from large cities will not interfere and where the least number of cloudy days will be experienced. A good telescope cannot be kept in a heated dome in winter, because the warm air escaping through the opening will produce a wavering of air which makes observations impossible, while variations in the density and curvature of the objective lens due to temperature variations will produce even greater errors.
TELESCOPES 63 The image formed by a simple lens is subject to a number of defects, called aberrations; for example, light of different colors is refracted by different amounts with the result that these different colors are Fig. 5. The 24-inch reflecting telescope of the Yerkes Observatory. (An astronomical photograph from the Yerkes Observatory, reprinted by permission of the Chicago University Press.) not brought to a focus in the same plane. Another type of aberration, known as spherical aberration, is the blurring of an image toward the edge of the field of view. Both of these aberrations may be reduced