Mr. Alvan Clark, of Cambridge, Massachusetts, has constructed an achromatic telescope, said to be the largest in the world. The glass has a focal distance of 23 feet, and a diameter of 18 inches. The glass revolves the sextuple star Theta, on Orionis, which is regarded as a severe test. Mr. Tuttle of the Boston Society of Natural History, tells that further has been disclosed by it.
“The telescope was then directed to the bright star Sirius, the most brilliant star in the whole heavens, and popularly known as the ‘Dog Star.’ All eyes eagerly sought for the minute companion star which Mr. Clark had discovered of his first directing the telescope to that star. It required but little time for those accustomed to look at minute telescopic objects to discern the little stranger hovering close within the dazzling rays of the brilliant Sirius, and almost exactly following it in right ascension. Its minute, tremulous light, when once caught by the eye, was easily kept in view, and as the star ascended in altitude, became still better seen. In piont of brilliancy, it does not differ greatly from the well known ‘sixth star in the trapezium’ of Orion.”
This telescope has removed both difficulties of spherical and chromatic aberration, and shows the images of the stars as round uniform disks, without any wings or wisps of light distorting their aspect.
Since the discovery of the companion of Sirius by Mr. Clark, Prof. Bond of the Harvard Observatory has seen it with his refractory telescope. He says:
“The low altitude of Sirius in this latitude, even when on the Meridian, makes it very difficult to catch sight of the companion, on account of atmospheric disturbances; when the images are tranquil, however, it is readily seen. It must be regarded as the best possible evidence of the superior quality of the object glass, that it has served to discover this minute star so close to the overpowering brilliancy of Sirius. A defect in the material of workmanship would be very sure to cause a dispersion of light, which would be fatal to its visibility.
It remains to be seen whether this will prove to be the hitherto invisible body long disturbing the motions of Sirius, the existence of which has long been surmised from the investigations of Bessel and Peters upon the irregularities of its proper motion in right ascension.”
– Published in The Burlington Weekly Hawk-Eye, Burlington, Iowa, Saturday, April 5, 1862, p. 3