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ASTRONOMICAL PHOTOGRAPHY 299
down the large telescope which is placed in a vertical position.
Unlike an ordinary telescope, this Tower Telescope, as it is called, has no tube, but the objective is simply held in position on the top of a light steel framework tower. (See fig. 127.)
A 12-inch objective of 60 feet focal length lies in a horizontal position on the top of this tower, and the sunlight is reflected into it from the second of the two plane mirrors. The first of these mirrors is mounted as a coelostat, and is rotated by an accurate driving clock at such a rate as to counteract the sun's apparent motion.
An image of the sun, 6*6 inches in diameter, is formed on the slit of the spectroscope 60 feet below, and 3 feet above the level of the ground.
The portion of the sun's image which it is desired to examine is allowed to fall on the slit, and the light of that portion passes through the slit, and descends vertically into a well about 30 feet deep. At a distance of 30 feet from the slit a 6-inch objective is placed, and the rays passing through that are rendered parallel. They then fall upon a Rowland plane grating, ruled 14,438 lines to the inch, and by means of this the light is broken up into a series of spectra.
The rays are returned through the same objective^ and the spectra formed are thus brought to a focus close to the slit.
When the grating is inclined at a very small angle, the images of the spectra fall a little to one side of the slit and can there be photographed. By this means portions of the spectrum 17 inches in length can be photographed at one operation.
When spot spectra are being photographed, only light from the darkest part is allowed to fall on the slit. At the end of the exposure that portion of the slit is covered, and light from the photosphere at a little distance from the