Abney, William de Wiveleslie, Sir, 1843-1920, Longmans, Green, and Co., publisher · London ; New York : Longmans, Green, and Co. · leaf 237
have — /- 20 X 20 400 , f= = !—• = 10-526. 40-2 38 ^ The diaphragms for this combination occupy a posi- tion half-way between the symmetrical lenses, and there- fore give no distortion. This lens covers an angle of about 60°, The next lens, fig. 58, is what is known as a ' wide angle ' doublet, in… … phragm is replaced by a 3rd compound meniscus lens. There were certain advantages connected with this lens at the time when it was introduced, but, since the manufacture of non- distorting doublets giving
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