In the spring of the following year, 1883, the attack upon the corona was carried on with a more suitable apparatus. The Misses Lassell were kind enough to lend me a seven-foot Newtonian telescope made by Mr. Lassell, which possesses great perfection of figure and retains still its fine polish. For the purpose of avoiding the disadvantage of a second reflection from the small mirror, and also of reducing the aperture to three and a half inches, which gives a more manageable amount of light, I adopted the arrangement of the instrument which is shown in the following woodcut:
The speculum b remains in its place at the end of the tube a, a, by which the mechanical inconvenience of tilting the speculum within the tube as in the ordinary form of the Herschelian telescope is avoided.
The small plane speculum and the arm carrying it were removed. The open end of the tube is fitted with a mahogany cover. In this cover at one side is a circular hole, f, three and a quarter inches diameter, for the light to enter; below is a similar hole over which is fitted a framework to receive the "backs" containing the photographic plates, and also to receive a frame with fine-ground glass for putting the apparatus into position. Immediately below, toward the speculum, is fixed a shutter with an opening of adjustable width, which can be made to pass across more or less rapidly by the use of India-rubber bands of different degrees of strength. In front of the opening f is fixed a tube, c, six feet long, fitted with diaphragms, to restrict as far as possible the light which enters the telescope to that which comes from the sun and the sky immediately around it. The telescope-tube a, a, is also fitted with diaphragms, which are not shown in the diagram, to keep from the plate all light, except that coming directly from the speculum. It is obvious that, when the sun's light entering the tube at f falls upon the central part of the speculum, the image of the sun will be formed in the middle of the second opening at d, about two inches from the position it would take if the tube were directed axially to the sun. The exquisite definition of the photographic images of the sun shows, as was to be expected, that this small deviation from the axial direction, two inches in seven feet, does not affect sensibly the performance of the mirror. The whole apparatus is firmly strapped on to the refractor of the equatorial in my observatory, and carried with it by the clock-motion.
