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Celestial Objects for Common Telescopes, Volume Two: The Stars
Celestial Objects for Common Telescopes Volume Two The Stars Author:Thomas W. Webb Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: scratches, in object-glass or speculum; they merely obstruct a very little light. Actual performance is the only adequate test. The image should be neat and well... more »-defined with the highest power, and should come in and out of focus sharply; that is, become indistinct by a very slight motion on either side of it. A proper test-object must be chosen ; the Moon is too easy : Venus too severe except for first-rate glasses ; large stars have too much glare; Jupiter or Saturn are far better; a close double star is best of all for an experienced eye; but for general purposes a moderate-sized star will suffice; its image, in focus, with the highest power, should be a very small disc, almost a point, accurately round, without ' wings,' or rays, or mistiness, or false images, or appendages, except one or two narrow rings of light, regularly circular, and concentric with the image :' and in an uniformly dark field ; a slight displacement of the focus either way should enlarge the disc into a luminous circle. If this circle is irregular in outline, or much brighter or fainter towards the centre," or much better defined on one side of the foctis than the other, the telescope may be serviceable, but is not of high excellence. The chances are many, however, against auy given night being fine enough for such a purpose, and a 1 The real diameter of a star in the telescope would be inconceivably small. The apparent or ' spurious' disc, and rings, results from the undulatory nature of light. They seem, however, to bo somewhat affected by atmospheric causes. Herschel II. speaks of nights of extraordinary distinctness, in which ' the rings are but traces of rings, all their light being absorbed into the discs.' I have entered 1852, March 23, as 'a very fine night, though the rings and appendages arou...« less