The physiological action of light / Professor James Dewar, F.R.S.E.

  • Dewar, James, Sir, 1842-1923.
Date:
[1876]
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Licence: Public Domain Mark

Credit: The physiological action of light / Professor James Dewar, F.R.S.E. Source: Wellcome Collection.

Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.

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    obtained—no doubt weaker, but still a current sufficiently strong for detecting any variation which light may produce. In these circum- stances, light still produces the variation I have described. To make it more definite still, pick out the retina with a fine glass point, and leave only the sclerotic and perhaps fragments of the choroid. Even then an electrical current is obtained; but this current is not affected by light. It is, therefore, proved that the variation produced by the action of light is due to some change or other occurring in the retina when light impinges upon it. Bays of the Spectrum.—Which rays of the spectrum produce the greatest effect? We know, of course, that the rays which are the most luminous to our consciousness are the yellow rays. The colours of a very pure spectrum were obtained, the eye being brought into the various rays successively, and the result noted. To obtain comparative results, the operations were repeated as quickly as possible. It was found, in studying the results, that those rays which we regard as the most luminous produce the greatest variation. For instance, the low red rays at the end of the spectrum produce very little effect, and if you go below the red into the heat rays there is no action. But the effect increases till you reach the yellow, and if you go on to the violet it gradually becomes less and less until, beyond the violet, there is no action. Belation of Electrical Variation to Luminous Intensity.—Experiments made have conclusively shown that a quantity of light, one hundred times in excess of another quantity, only modifies the electric variation to the extent of increasing it from three to five times its original amount. The effects observed vary in such a manner as to correspond closely with the relative variations that would result if the well- known psycho-physical law of Fechner was applicable to this class of phenomena. The effect of Fatigue.—The retina, on the action of light, behaves in a similar manner as regards fatigue, to a muscle that has been exhausted by repeated stimulation. The muscle diminishes in its mechanical effect for the same stimulation and recovers during repose. The amount of electric variation in the case of the eye diminishes for the same amount of light stimulation, unless the organ has had sufficient time to recover its normal condition. In this case, the recovery takes place in the absence of light. We have continued this investigation in various new directions, and have arrived at results which may be thus shortly detailed. New Method, of Experimenting.—One of the chief difficulties in arriving at the exact relation between the electrical variation and the different luminous and colour intensity of light, was the con- tinually diminishing sensibility to the stimulus, owing to the abnormal conditions of the eye when removed from the head. You can easily understand how this occurs. When you begin the experiment, the eye is remarkably sensitive to light, and a large variation of current is obtained; but the amount of this current is gradually falling in
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