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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    Bogal institution of ©teat Britain. WEEKLY EVENING MEETING, Friday, March 31, 1876. George Busk, Esq. F.R.S. Treasurer and Vice President, in the Chair. Professor James Dewar, F.R.S.E. Part II.* The Physiological Action of Light. On a former occasion I communicated the results of a research on the “ Physiological Action of Light ” executed conjointly with Professor T. G. McKendrick. The principal facts then established may he thus summarized. Comparative Physiology of the Action of Light.—The ^impact of light upon the eyes of members of the following groups of animals, viz. mammalia, birds, reptiles, amphibia, fishes, and crustaceans, produced a variation amounting to from three to ten per cent, of the normal current. At that time we found light caused a negative varia- tion in the case of warm-blooded animals. Transmission of Action to Brain.—The electrical variation mav be traced into the brain. Instead of severing the eye from the brain and cutting the optic nerve, simply remove the head of the frog. Then suppose one of the electrodes in contact with the surface of the brain and the other in contact with the surface of the cornea, an effect is obtained from the action of light similar to that just described. Action due to Change in Betina.—This action is really due to an alteration in the retina itself. This must be definitely proved, because it was a legitimate criticism that the change produced by the action of light may be due to contraction of the iris; the iris being a muscular structure contracting on the action of light by a well-known reflex mechanism in normal circumstances, and even after removal of the eye from the head. A contraction of the iris might pro- duce a negative variation or diminution of the electrical current, but it is difficult to imagine that it could cause an increase or positive variation. In order, however, to get rid of this difficulty, cut off the front part of the eye altogether, and place one electrode so as to touch the surface of the vitreous humour while the other impinges on the transverse section of the optic nerve, a current is * See R. I. Proceedings, vol. vii. p. 300. B