March 15, 1877] 



NATURE 



433 



THE PHYSIOLOGICAL ACTION OF LIGHT ^ 



T^EW Method of Experimenting. — One of the chitf 

 "* ^ difficulties in arriving at the exact relation between 

 the electrical variation and the luminous and colour in- 

 tensity of light, was the continually diminishing sensibility 

 to the stimulus, owing to the abnormal conditions of the 

 eye when removed from the head. When the experi- 

 ment begins, 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 consequence of the 

 gradual change in the parts of the eye, owing to their loss 

 of vitality and sensibility. In fact, the parts are dying-- 

 the blood is not circulating, and molecular and chemical 

 changes are slowly occurring. In the case of the frog 

 however, it is a fact that the retina retains ics sensi- 

 bility from three to four hours, and sometimes longer. 

 After a lapse of two hours the frog's eye frequently re- 

 mains in a tolerably stable condition, in which it d oes not 

 lose sensibihty rapidly. This condition may last for four 



¥ 



Didgram showing arrangement of apparatus in the experiment on eye o 

 frog A, Ej'e showing the electrode, E, in contact with it. B, Skin re- 

 moved, and subcutaneous tissue in contact with other electrode, K. K, 

 Key._ G, G.lvanoraeter. Arrows indicate direction of current. Cornea, 

 positive. Back, negative. 



or five hours. In order to get rid of the difficulty of 

 gradual death of the parts, various methods were tried 

 in earlier experiments in the attempt to remove the eye 

 as quickly as possible, and to make the observations 

 rapidly. In the case of the warm-blooded animals this 

 did not lead to good results, because the sensibility to 

 light disappeared in a very {f^ minutes. On several 

 occasions the posterior aspect of the eye was exposed in 

 the living anaesthetised warm-blooded animal, and on 

 bringing one electrode into contact with the severed optic 

 nerve while the other touched the cornea, the observa- 

 tions were tolerably constant. This method was trouble- 

 some and difficult. 



These experiments are now made in a different way. 

 By placing a frog, rabbit, or pigeon under the influence of 

 chinoline, the animal remains motionless, A small por- 

 tion of the surface of the cranium is then removed so as 

 to expose a portion of the brain. One of the electrodes 

 is brought into contact with the surface of the cornea, and 

 the other with the surface of the brain. The blood is 



' Fiiday evening Lecture by Prof. James Dewar, M.A., at the Royal 

 Institution, March 31, 1876. See Nati;re, vol. viii. p. 204. 



Still circulating. A current is obtained ; and all the 

 effects I have just mentioned may be observed with ease. 

 The animal remains in this condition, retaining its sensi- 

 bility to the action of light, for as long a period, in the 

 case of the frog, as forty-eight hours. These observations 

 led to the discovery made recently, that there is no 

 necessity for even exposing the surface of the brain. That 

 is to say, the action of light can be traced, if needful, 

 through the whole body. If, for example, we take a .'rog, 

 place it in position, slightly abrade the skin on the surface 

 of the head or back, or any part of the body, then adjust 

 the electrodes, one in front of the cornea and the other 

 upon theabraded skin, we obtain an electrical current which 

 is affected by light in the usual way. But if the electrode 

 in contact with the cornea be shifted to some other part 

 of the body, a current may be obtained ; but this current 

 is not sensitive to light. In order to produce the specific 

 action of light upon the eye, the retina must be included 

 in the circuit. This discovery enabled us to perform many 

 experiments without injuring the animal, except to the 

 extent of abrading or removing a smair portion of skin. 

 It at once opened up the way for making observations 

 upon warm-blooded animals (one of the chief difficulties 

 in our earlier investigations). For example : give a rabbit 

 or a guinea-pig a small dose of chinoline, and the animal 

 remains prostrate and quiet. Then cut off a little of the 

 hair from the surface of the head at the back of the neck, 

 and abrade the skin so as to have a moist surface ; bring 

 the electrodes into position, placing one in contact with 

 the abraded surface, and the other in contact with the 

 surface of the cornea, and you will at once obtain the 

 effect. 



Action of Light i?t Warm-blooded same as in Cold- 

 bloodea Anitnals. — By the use of chinoline we were" able 

 to make experiments of the kind just described for a con- 

 siderable time, without the necessity of maintaining arti- 

 ficial respiration. The result of those investigations upon 

 warm-blooded animals has been to show that in these, as 

 in the cold-blooded, light produces first an increase in the 

 electric current on impact ; continued light usually causes 

 the electrical current to diminish ; and on the removal of 

 light, there is a second rise, as described in the case of the 

 frog. In our earlier investigations, we always observed in 

 the case of warm-blooded animals (when the eye had either 

 been quite removed from the body or was receiving an in- 

 adequate supply of blood), that the action of light caused a 

 negative variation, that is, a diminution in the electrical 

 current. By improved methods, however, which have the 

 effect of placing the eye in conditions more normal, we 

 find that light causes a positive variation, that is, an in- 

 crease ; thus agreeing with what had hitherto been 

 observed in the eye of the frog. This is a point worthy 

 of notice. Du Bois-Reymond showed, even in the case 

 of sensory nerves, that physiological action caused a 

 negative variation. But it appears that in the case of the 

 retina the action of the normal stimulus is to cause a 

 positive, not a negative variation. 



Experiment with the Living Lobster. — The action of 

 light can be readily shown in this animal. Fix it loosely 

 in a cloth, and lay it on the table in a slightly oblique 

 position. With a small trephine remove a circular por- 

 tion of the carapace, about three millimetres in diameter, 

 and expose the moist tegumentary surface. Bring one 

 electrode into contact with this surface, while the other 

 touches the cornea. The usual effects of light may then 

 be noted ; but in the case of the lobster, the variation 

 caused by the impact is greater than what we have noticed 

 in any otlier animal, often amounting to one-tenth of the 

 total amount of current. Another interesting experiment, 

 comparable with that of the two eyes just described, may 

 be made on the lobster by placing an electrode in contact 

 with each cornea. The result frequently is apparently no 

 current, but in reality the currents neutralise each other. 

 Light falling on the one eye causes the needle to move, 



