Synopsis
This historic book may have numerous typos, missing text or index. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. 1917. Not illustrated. Excerpt: ... is cooler, but seldom sufficiently thick. The Germans have employed deep yellow. Herschel I. adopted, with great success, a trough containing a filtered mixture of ink and water. Cooper, at Markree Castle, Ireland, used a * drum' of alum water and dark spectacles, and could thus endure the whole aperture, 13-- inches, of his 25-foot achromatic.1 With large apertures, a plane surface of unsilvered glass placed diagonally, as originally suggested by Herschel II., so as to reflect only a small fraction of the light and heat, is found of eminent service. Merz, of Munich, has so reduced the light by polarisation at four such surfaces, that a dark glass, the tint of which is of course better dispensed with, becomes unnecessary; and the same end is attained in a very ingenious double-prism eye-piece devised by Prof. Pickering: neither apparatus, however, is free from accidental colour.2 An eye-piece constructed by Andrews with two lenses of complementary tints, burnished in loosely to avoid fracture from expansion, has succeeded with a small aperture. In screen-glasses combinations of colour are good. Bed succeeds perfectly with green, or with green and blue. Herschel II. used green and cobalt blue.3 A Barlow lens, carefully silvered, is said to act admirably, though a light screen might still be necessary. If there is to be only one solar cap, deep bluish-grey, or neutral tint, will be quite satisfactory; if several, it would be worth 1 Not, however, an example to be imitated. Dawes thought that with a focus of 30 inches, 2 inches of aperture were enough for perfect security. A 4J-inch silvered mirror is not safe for screen-glasses. 2 An unsilvered glass mirror has been found an excellent instrument for solar observation.--(P.) * The value of complementary, or at any ...
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