Optical tables and data for use of opticians / by Silvanus P. Thompson.

  • Silvanus P. Thompson
Date:
1907
Catalogue details

Licence: In copyright

Credit: Optical tables and data for use of opticians / by Silvanus P. Thompson. Source: Wellcome Collection.

    49/148 page 37
    Multiplying by 0-52 we get 13 dioptries as the convergivity or power of the lens. The focal length is therefore metre (= O'0769 metre or 3'03 inches). ^he curvature of a convex surface is reckoned positive, that of a hollow or concave face, negative. In the case of any lens that has a concave face with radius of curvature r, the curvature of that face would be reckoned as — i. r A further illustration is afforded by the calculation of the distances of conjugate foci. Suppose light to be proceeding from a point P on the left. Its wave-surface as it spreads will diverge, the radius of it being the distance back to the point P. Call this distance u. At that distance from P the wave-surface will have a curvature which will become smaller as the distance u increases. Thus if distance u reckoned back to P is 200 milli- Fig. 5. metres or 0 • 200 metre, the curvature of the wave-front will be — 5 dioptries. At the distance 0-333 metre the curvature will — 3 dioptries, and so forth. The negative sign means that this wave has a divergent surface.
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