Wilson's quarter century in photography - a collection of hints on practical photography which form a complete text-book of the art (1887) (14591647999)

Identifier: wilsonsquarterce00wils_0 (find matches)
Title: Wilson's quarter century in photography : a collection of hints on practical photography which form a complete text-book of the art
Year: 1887 (1880s)
Authors: Wilson, Edward L. (Edward Livingston), 1838-1903
Subjects: Photography
Publisher: New York : E.L. Wilson
Contributing Library: Getty Research Institute
Digitizing Sponsor: Getty Research Institute



Text Appearing Before Image:
ABOUT LENSES. 39 respective radii extending from thirty feet down to a hundredth of an inch.The curvature to which any particular lens is to be ground is calculated mathe-matically to suit the refractive and dispersive ratios of the glass of which it isto be formed ; and after the lens is finished, if, on examination, it fails to comeup to the standard of sharpness, the particular surface which exercises control•over the shortcoming is reground in a tool one degree deeper or shallower incurvature. Fig. 14. Fig. 15.
Text Appearing After Image:
A large shop in every factory is devoted to brass-turning and fitting. Inthe engraving, Fig. 15, is seen one of the numerous workmen engaged in afterwards, is an important point. But let us go on in the same way, still reducing thatsurface by making it a concave or negative one; it is clear that the optical centre stillmarches on, moving out of the lens, and if we go on so far as to make the negative curveequal to the positive one, then the optical centre would be in infinity, and if we disregardthe thickness, we have no lens, but a non-optical glass like a watch-glass. All straightlines passing through the optical centre of a lens are called secondary axes. The nextand most important of the general properties of a lens are its principal focus and theconjugated foci. If we hold a convex lens towards the sun, and a sheet of paper at acertain distance behind it, we observe a bright little circle, in which the sunlight, fallingupon the lens, is collected; the point where the circle is sm