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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![Let a = ha = h'a—ma nearly. X = h e = V e = m e nearly, a = the angle h am = V am. j3 — the angle bem = b' e m. Then, in Fig. 1 for the convex lens, d . hm d . r> ^ tan. a = = - ; tan. p = = - am 2 a e m 2x And in order that the prism may be adapted to the lens, we have the angle of deviation xp = a — /3, A = tan. lb =tan.(a—/3) — “ ~ —tan.a —tan./3 ^ 1 + tan. a tan. /3 — r — ~ j since the angles are small. 2 V ® ^ J But, from formula [1] for the lens, we have — = (w — 1) i ; where, since the emergent rays diverge, the negative sign is given to x; and, since the second surface is 1 plane, r=oo , — = 0. 7i = -f-r 1-—1). 1 = 2 V « X ) 2 ^ ’ r 1 ■ / ’ or, if the angle of the prism be required. h d — ; as in pages 6 and 7. ! n — \ 2 (n — 1) The same investigation would apply to figure 2 for the con- cave lens, except that /would then be negative, which would make m also negative, which would imply that the base of the prism must be turned in the contrary direction fi'om the frontal axis. 4. To find the distance from the centre of a larger lens, at which smaller prismatic lenses may be cut eccentrically. In Fig. b, if c be the point where the ray p p' crosses the](https://iiif.wellcomecollection.org/image/b2236397x_0020.jp2/full/800%2C/0/default.jpg)
