The theory of ocular defects and of spectacles / translated from the German of Hermann Scheffler by Robert Brudenell Carter with prefatory notes and a chapter of practical instructions.
- Hermann Scheffler
- Date:
- 1869
Licence: Public Domain Mark
Credit: The theory of ocular defects and of spectacles / translated from the German of Hermann Scheffler by Robert Brudenell Carter with prefatory notes and a chapter of practical instructions. Source: Wellcome Collection.
Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.
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![fectly general, and can be adapted to concave surfaces by making one or both of the radii negative; to plane surfaces by making one or both of them infinite; to parallel rays, before or after refraction, by making a or x infinite ; to con- vergent incident rays by making a negative; to divergent emergent rays by making x negative.* To adapt it to rays parallel before refraction ; Let a = 00 , -L = 0; and for x substitute f, as it will now become the principal focal length of the lens ; JL =i {n — 1) ( - + —^7 ^ for a bi-convex lens. / V r r J 1) + i),for a bi -concave lens, since / in the latter case r' and r’’ are both negative. From the former f= r r' , as in page 8 ; and. {n — 1) (/•' -1- r) by substituting this value in formula [1], we have — -f- i , for converging rays after refraction ; a X f i i = ^ , for diverging rays. If r is negative, and > r', then we have the convexo-con- cave lens mentioned in p. 230, for which the formula becomes p // _ 1) {r _ r’)' 1 1 Again, from the formula — + — = _ , we have a X f a — f “ ~ ^ * Various assumptions have been made by different writers about the signs of these quantities; that of Mr. Coddington {Bejlexion and Befraction, § 85) has been here adopted, as it makes the formulse harmonize in this respect with those in the following pages. a](https://iiif.wellcomecollection.org/image/b2236397x_0017.jp2/full/800%2C/0/default.jpg)
