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Lens Story: 6 of 28
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·Back Cover
Eye-end of Lick telescope
Lick Observatory, founded by James Lick, is situated on
the summit of Mt. Hamilton, near San José, California
and it is always upright. An ordinary mirror gives an example of a virtual image. Both types of images are repeatedly illustrated in optical instruments, as will be seen.
    There are two main types of lenses, the converging and the diverging. The convex lens belongs to the former class and the concave lens to the latter. As shown in the figure of on page 2, the double convex object glass of Galileo's telescope converges the rays of light from the object AB tending to produce a real image at ab. Before the rays reach this position, however, they are diverged by the concave eyeglass and are therefore made to produce a magnified virtual image at A'B'. The eyepiece magnifies by apparently increasing the visual angle, and, thus, by causing a distant object to appear larger, it seems to be nearer.
Refracting Telescopes
    From Galileo until very recent years the refracting telescope has maintained the ascendancy, and it has contributed most to the advancement of astronomical science. But, very early in the making of telescopes, a serious obstacle presented itself. Sir Isaac Newton discovered that prisms and lenses bend rays of different colors unequally. The violet rays are refracted most and the red least. Therefore the violet rays are brought to focus nearer the object glass than the red rays and the whole image is surrounded with a fringe of color. But by grinding lenses almost flat and of very great focal length this difficulty was largely overcome.
Transporting the 100-inch reflector for the solar observatory of the Carnegie Institution

To the top of Mt. Wilson (6,665 feet), near Pasadena, California
Another and ever more serious difficulty, however, immediately appeared, for these flat lenses necessitated very long and unwieldy telescopes awkward to manipulate and requiring a prodigious amount of time and patience in their use. Telescopes were built over two hundred feet in length and with no tube connecting object glass and eyepiece. In one such telescope built by Huygens the object glass was mounted in a small swivel on the top of a tall pole and secured in position by a rope held taut by the observer, who also held the eyepiece in his other hand. Needless to say very little useful observation could be made with such a telescope.
    In 1733 Chester More Hall and later, in 1755, John Dollond, two English