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.

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    red. This is further a function of the size of the surface, for on diminishing the surface areas of both of the now unequally bright fields, a limit is reached, when both appear again equally bright.1 Usually, in comparing two li^ht sources of considerably different colour, the difference is removed or lessened by cutting out the same spectral region by coloured glasses and measuring for this. But obviously it is not legitimate to deduce the ratio of the total intensities by this method. M. de Lepinay 2 3 uses two light-filters, one transmitting red, the other green, and compares the lights for both. If R is the ratio for red, G for green, and N for the total intensities, according to Ldpinay R ( G\ — = i + o'2o8^i — — j. A similar method is used by Weber in his photometer,15 but this is only a slightly better approxima- tion, in view of the great variation of the spectral emission curves of different light-sources. Siemens and Weber have proposed methods depending on visual acuteness, but the results are not satisfactory. Flicker Photometers.—It is claimed for flicker photo- meters that the measurements are independent of the Purkinje phenomenon. Talbot’s law is valid not only for rapidly con- secutive illuminations of different intensity, but also when these are qualitatively different. If a surface is alternately lit by lights of different colour and intensity, a mixed colour is seen when the rate of alternation is sufficient, otherwise the sensation known as “flicker” is produced. Now this limit is attained for a smaller number of periods according as the difference of intensities is less. Hence, by adjusting the intensities till the 1 This dependence on the area is due to the fact that for the fovea centralis and the macula lutea—the rodless region of the retina—the Purkinje phenomenon is absent (J. von Kries in W. Nagel’s Hdbuch. d. Physiologic, p. 182 (1904)). K. Schaum (Zeit. f. IViss. Phot., iii.’272 (r 905)) suggests that the curve of spectral brightness for a mean intensity is produced by summation of curves for a very high and very low intensity, corresponding to the curves for the rods and cones separately. 2 C. R., 97, 1428 (1883). 3 Zeitschr.f. Elektrotcchnik, loc. cit. (1889).
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