520 Royal Society : — 



object from the variety of its course, as into each anterior ciUary vela 

 the blood is seen pouring out from the circular vein in two opposite 

 currents, to be united into one in the larger vessel. The author also 

 describes the appearance presented by the blood poured into the 

 circular veins by their afferent vessels. 



" Researches on the Action of certain parts of the Solar Spectrum 

 upon the Iris." By E. Brown Sequard, M.D. 



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, 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. 

 It seems therefore that it is not by a chemical action, but by a 

 peculiar dynamical influence that light produces contraction of the 

 iris. 



The power of the iris to contract when stimulated by light lasts 

 extremely long, particularly in certain animals. In the eel I have 

 found the muscular irritability of the iris, in one case, lasting sixteen 

 days, during the last winter, in eyes taken out from the orbit. This 

 is an interesting fact, not only on account of the long duration of 

 vitality in the iris, but on account of the conclusion that we are 

 entitled to draw from it, that muscular fibres may be stimulated 

 without the intervention of nerves. In the iris of the eel the nerve- 



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

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



