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: ... Large Eruptive Prominence Photographed At The Solar Physics Observatory, Cambridge, 1915, April 19, At Io1143"'And Ii11 41m. (By kind permission of the Royal Astronomical Society.) while to have different colours, Secchi's observations at Rome seeming to show that the visibility of very delicate details may depend on the tint. In the absence of a proper screen, smoked glass may be used: it is said to intercept heat very perfectly, by Prince, who places it within the eyepiece, close to the 'stop,' or circular opening, which bounds the field; but thus it can have only one degree of depth, and must be taken out to view other objocts. A strip of glass may be smoked to different densities in different parts, and held between the eye and eye-piece; but it should be protected from rubbing by a similar strip of glass placed over it, and kept from touching by bits of card at the corners, the edges of the two strips being bound round with gummed slips of paper, or tape.1 Where expense is not regarded, an optician will provide a delightful graduated screen with two wedges of glass, plain and coloured. A more complete command may be obtained by two such wedges sliding behind one another in a brass cap. In any case we should not begin with too faint a shade, bnt try the deepest first, and change it if necessary. The thickness of any external screen will contract the field much, unless the eye is brought as close up as possible. A pleasanter view is obtained by placing the screen within the adapter, but from its larger surface it will require to be worked very true. To bring the Sun into the field, do not attempt to look for it with the finder unless it has a solar cap: point the telescope till the finder shows it centrally on the hand, or on a paper held behind it; or bring ...
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