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II. "Researches on the Action of certain parts of the Solar 

 Spectrum upon the Iris." By E. BROWN SEQUARD,, M.D. 

 Communicated by Dr. SHARPEY, Sec. R.S. Received 

 July 10, 1856. 



In 1847 I discovered that light has the power of acting directly 

 upon the iris so as to produce there a muscular contraction, mani- 

 festing itself by the constriction of the pupil. If an eye taken out 

 from the orbit is alternately exposed to light and darkness, we find 

 that the pupil becomes alternately constricted and dilated*. 



It was interesting to know whether the stimulation of the mus- 

 cular fibres of the iris is produced by the chemical power of light or 

 not. I had already found, in 1847, that only the parts of light 

 which seem to have but very little chemical action, have the power 

 of exciting contractions in the iris. But my experiments having 

 been made with light passing through coloured glasses, were not 

 decisive. Lately I have performed many other experiments in 

 making use of light decomposed by the prism. In one case, with 

 the assistance of Messrs. Dubosc and Nachet, jun., I experimented 

 with electric light, and in the other cases I made use of direct 

 solar light. 



In all these cases the same results have been obtained. I uni- 

 formly found that the yellow part of the spectrum acted as well as 

 undecomposed light, and that the other parts of the spectrum had 

 either no action at all, or only a very slight one. The parts of the 

 green and orange adjoining the yellow had a decided but very slow 

 action. The two extremities of the spectrum, and the dark places 

 in their neighbourhood, not only had no constrictive action upon 

 the pupil, but did not prevent it from dilating, and the dilatation 

 seemed to take place as quickly as when the eye was put in complete 

 darkness. 



From these experiments it appears that the power possessed by 

 light, of stimulating the circular fibres of the iris, belongs not to its 

 chemical or to its calorific parts, but to its illuminating elements. 



* Comptes Rendus del'Acad. des Sciences, vol. xxv. pp. 482 & 508 ; and Comptes 

 Rendus de la Societe de Biologic, vol. i. p. 40. 



