Page:The Moon (Pickering).djvu/127

Rh which the light before it reaches the photographic plate is reflected from two mirrors, one in front of and one behind the object glass. The aperture is 23.5 inches and the focus 62 feet. The lens is corrected for the photographic rays, and is usually reduced in aperture to 21.3 inches (54 cm.) when in use. The focal length is redetermined every month. The diameter of the Moon on the original negatives, which are subsequently enlarged, is about 6.7 inches. The most rapid plates are employed, and the exposures vary from 0.5 to 4 seconds. During the exposure the telescope remains stationary, while the plate is moved by clockwork. The total number of photographic negatives already secured is about 400. This monumental work, showing the very best photographic results of the nineteenth century, may very properly bring our history to an end.

Our knowledge of the surface of our satellite is now so complete that while we do not know any portion of it as well as we do large tracts in Europe and the United States, yet at the same time there is no portion of its surface that is readily visible that is not known far better than large areas of the readily accessible surface of our own globe.