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its falling: and it is in other respects convenient to work with. Hence several large telescopes have already been built in this way. A good example is the Snow telescope on Mount Wilson. You see in the picture the long shed which is really the telescope itself: and at one end there is the coelostat mirror, reflecting the light (in this case usually the light of the Sun, not that of a star) into the telescope.

But there is one great disadvantage about this horizontal position. The ground gets heated during the day and causes air currents to ascend from it. Now currents of heated air blur the image (see p. 143) and are to be avoided as much as possible, whereas the horizontal position of the telescope seems to encourage them. Hence, Professor Hale tried the plan of putting the telescope vertical instead of horizontal. He built, in the first instance, a tower 60 feet high. The coelostat mirror[1] was placed on the top and the rays from the Sun or stars sent down to the foot of the tower, where they could be examined or photographed. This new move was found to be so successful that a more ambitious experiment was tried. A tower 150 feet high has been built to carry the coelostat at the top: by it the rays of the Sun are reflected down to the ground but do not stop even there, because a well 80 feet deep has been dug below the tower to receive them, so that the whole length of travel

  1. There are really two plane mirrors, one moved by clockwork as already described, the other fixed. If we had only the first mirror, then any part of the sky would indeed remain stationary, but the telescope must be pointed to it in a particular direction. The second mirror makes it unnecessary to point the telescope.