Photo-chemistry / by S.E. Sheppard.

  • Samuel Edward Sheppard
Date:
1914
Catalogue details

Licence: In copyright

Credit: Photo-chemistry / by S.E. Sheppard. Source: Wellcome Collection.

    31/484 page 15
    definitions of the quantities involved.< We cannot deal abso- lutely with the energy radiated without at the same time con- sidering the recipient surface. Further, in considering the three methods of measurement, it is to be noted that the eye cannot make absolute judgments of the amounts of light radi- ated by different sources. But if two surfaces which are sepa- rately illuminated are made optically contiguous, then the eye can distinguish very small differences in the brightness of the surfaces. On this faculty depends the methods of photometry, which are essentially comparative. Brightness is a subjective quality, expressing simply the degree of the physiological stimulus due to light, which has of course its objective corre- lative in the “ photo-chemical effect,” whatsoever that may be, in the retina. Light-Sources.—All self-luminous bodies and also diffuse reflectors may be considered as light-sources proper. They may vary in size, in intensity, and in quality or colour of the light emitted. The ideal light-source, for simplicity of treat- ment, is a point, a good approximation to which is obtained by a hole pierced in a screen illuminated by a convenient source.1 The conditions of experimental work will determine how far any light-source may be treated as a point-source. Amount of Light.—Considering such an ideal point- source in free space, then the amount of light emitted is that which falls on a surface completely surrounding it. This we term A. Light-Strength.—A useful conception is that of a tube of light. This is a cone of rays whose apex is the point-source, and the ideal bounding walls of which are formed by light- rays. In every cross-section of this tube or cone the amount of light will be constant. This follows immediately, for a per- fectly transparent medium, if we consider that the amount of light is the energy transmitted per unit time through the cross-section of the tube. If we consider a sphere surround- ing the point as centre at a radius of 1 cm., the cone will cut oft’ a surface, <£, on this, the angle subtended by the cone. 1 Compare this use in Young’s celebrated experiment on interference. Preston, Theory of Light, 2nd ed., p. 24; Baly, Spectroscopy, p. 16.
    digitised image 1
    digitised image 2
    digitised image 3