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Ma
1923
'85
vived after the applications of photography, and still larger ones were constructed, notably the seventytwo-inch reflector at the Dominion Astrophysical Observatory at Victoria, B. C, and the one-hundredinch reflector at the Mount Wilson Observatory.
Every ray, of whatever color, is brought to the same focus on the plate with the reflector. For this reason, and others, they operate much faster than refractors of the same size. A first-class reflector of, for instance, thirty inches aperture could probably photograph, with prolonged exposures, everything that could be seen with a refractor of fifty inches aperture were it possible to construct a perfect objective as large as that.
Photographing with Refractors
Refractors, however, have their own field in which they are unrivaled, and it is interesting to note how they were adapted to make photographs on a large scale which could be used for the most refined measurements. Thus, when the forty-inch Yerkes refractor began its work in 1897, it was not expected that it would be used for direct photography. Within three or four years, however, it was found — chiefly through the efforts of G. W. Ritchey — that it could produce finer photographs than had been obtained up to that time, of such objects as the moon and the star clusters, which were adapted to the most precise measurement.
We do not know who is responsible for the first suggestion* of the procedure, but it is perfectly obvious after being suggested. It consists in using a yellow filter directly in front of the isochromatic plate, which is especially sensitive to yellow light. The refractor was designed for use with the eye and corrected so that the image would be in sharp focus for the rays which most affect the eye, namely, the red, yellow and green. Accordingly, as the filter cuts out the rays
REGION OF THE MILKY WAY NORTH OF THETA OPHIUCHI
Photographed by Professor Barnard with the Bruce telescope, with an exposure of three and a half houn
This region of the Milky Way is wonderful on account of its dark markings as well as its abundance of stars. The image of the bright star, Theta, in the lower part of the picture, is greatly enlarged by over-exposure. The photograph shows many dark markings, which probably represent absorbing matter — perhaps in the form of dark nebulae — lying between us and the Milky Way. Note the S-shaped dark figure below the center of the picture, just above Theta.
for which the telescope was not originally corrected, beautifully sharp images are obtained, and these on a very large scale because of the great length of the telescope (sixty-two and one-half feet).
the
The refractor thus wastes blue and violet rays, which are most effective on the ordinary photographic plate ; but nevertheless, photographs are obtained with sufficiently short exposure for the
THE CORONA AT THE TOTAL SOLAR ECLIPSE OF JUNE 8, 1918 As photographed at Matheson .Colorado, by Edison Pettit of the Yerkes Observatory
The cornoa is an external envelope of the sun visible only during a total solar eclipse. It contains a gas not yet found on the earth or in the stars, known as coronium. There is still much mystery about the corona because it can be so seldom observed, and then only for such a short interval.