March 22, 1877] 



NATURE 



45 



THE PHYSIOLOGICAL ACTION OF LIGHT ^ 

 II. 



£\ETERMINATION of Electro-motive Force.— Soon 

 ■^^ after the first experiments were announced, certain 

 physiologists said that although the results of the action of 

 light which I have just described may be observed, to say 

 there was a change in the electro-motive force, as stated in 

 the earlier communications, was not correct. That the 

 elTtct was due to an alteration in the electro-motive force 

 had been proved, but experimental details were reserved for 

 the second part of the investigations. At first Sir William 

 Vhomson's electrometer was used, but the amount of 

 ^.^;ctric potential to be measured was too small to get 

 feood results. Another plan of determining the electro- 

 motive force was adopted. This was the method intro- 

 duced by Mr. Latimer Clarke, the eminent electrician, 

 and described in his work on " Llectrical Measure- 

 ments." The instrument devised for this purpose is 

 called by him a Potentiometer, and measures electro- 

 motive forces by a comparison of resistances. Practicaly 

 we found the Daniell's cell far too strong a battery to use 

 as a standard of comparison. A thermo-electric junction 

 of bismuth and copper was substituted for it. One end 

 of the junction was constantly heated by a current of 

 steam passing over it, the other being immersed in melt- 

 ing ice. The electro-motive force of this thermo-electric 

 junction, as estimated many years ago by Regnault, is 

 extremely constant, and is about the ^fs^h part of a 

 Ddniell's cell. By means of this arrangement the follow- 

 ing results were obtained : — The electromotive force of 

 the nerve-current dealt with in experiments on the eye 

 and the brain of a frog varies from the rToo^h to the 

 j^j;th of a Daniell's cell. Light produced an alteration 

 in the electro-motive force. This change was, in many 

 instances, not more than the T(j^^(jth of a Daniell's cell. 

 But though small it was quite distinct, and proved that 

 light produced a variation in the amount of the electro- 

 motive force. By the same arrangement the gastroc- 

 nemius muscle of a well-fed frog gave ^^th of a Daniell ; 

 the same muscle from a lean frog which had been long 

 kept, gave ^^^th of a Daniell ; and the sciatic nerve of 

 the well-fed frog j|pth of a Daniell. Dr. Charles Bland 

 Radclifife states, in his " Dynamics of Nerve and Muscle," 

 p. 16, that he obtained by means of Sir William Thomson's 

 quadrant electrometer, from a muscle a positive charge 

 equal to about the tenth of a Daniell's cell, a much greater 

 amount than ascertained by the method I have just de- 

 scribed. 



The electro-motive force existing between cornea and 

 posterior portion of the sclerotic in a frog amounts to 

 ^^gth part of a Daniell, and between the cornea and 

 cross section of the brain is about four-fifths of the above. 



Effect of Temperature on the Eye of the Frog. — From 

 numerous experiments on the irritability of muscle in- 

 duced by the excitation of nerve, it has been satisfactorily 

 proved that a temperature of about 40° C. destroys the 

 action of motor nerves in cold-blooded animals. Up to 

 the present time we are acquainted with no observations 

 as to the temperature at which a terminal sense organ be- 

 comes incapable of performing its functions. Having 

 satisfactorily proved that the retitia is the strttcture in 

 the eye producing the electrical variation observed, it be- 

 comes evident that as long as this phenomenon can 

 be detected the retina is still capable of discharging its 

 normal functions. In order to investigate thoroughly the 

 effect of an increasing temperature on the sensibility of 

 the retina, a method of procedure was adopted of which 

 the following may be taken as a general account : — 

 A frog was killed, the two eyes removed rapidly from the 

 body ; the one eye was placed on electrodes and main- 

 tained at the ordinary temperature of 16° C, while the 



' Friday evening Lecture by Prof. James Dewar, MA., at the Royal 

 In.nitution, March 31, 1876. See Nature, vol. viii. p. 204. Continued from 

 p. 43S- 



Other was placed on similar electrodes contained in th:; 

 interior of a water bath having a glass front, the sides of 

 the air chamber being lined with black cotton wool satu- 

 rated with water. Into this chamber a delicate thermo- 

 meter was inserted, and the currents coming from the two 

 eyes were alternately transmitted to the galvanometer every 

 five minutes by means of a commutator, the temperature 

 and the electrical variation produced by the same amount 

 of light being noted in each case. The general results are 

 shown in the following table : — 



Table showing Comparative Effect of Temperature on 

 Sensibility of Frog's Eye. 



The initial amount of current was, however, increased 

 on the whole by the action of the higher temperature, 

 thus showing that the sensibility to light does not depend 

 on the amount of current circulating through the galva- 

 nometer. It will be observed, on inspecting this table, 

 that the eye maintained at the temperature of 16"^ C. 

 remains tolerably constant in its initial action, although 

 it gradually gets more sluggish, whereas the final effect 

 steadily rises. On the other hand, in the case of the eye 

 subjected to a higher temperature, the initial effect seems 

 to have a maximum about 29° C, then gradually dimi- 

 nishes, and vanishes about 43° C, the final effect con- 

 tinuously falling and being actually reversed. To succeed 

 in this experiment it is necessary to heat the electrodes 

 which are to be used in the water bath up to 40° C, in 

 order to be certain that no changes are induced in the 

 electrodes themselves that might be mistaken for those 

 above mentioned. An eye that had been placed in dilute 

 salt solution along with lumps of ice was found to have 

 the usual sensibility to light. 



Effect of Temperature on the Eye of Pigeon. — Having 

 succeeded in experimenting with a water-bath in the 

 manner above described, it appeared interesting to ascer- 

 tain if the eye of a warm-blooded animal would be bene- 

 fited by being maintained at the normal temperature of 

 the body. The head of a pigeon was placed in the water 

 bath at a temperature of 40° C, the eyes were ftund 

 sensitive to light, the action, however, being alwajs a 

 negative variation ; but instead of vanishing quickly, as it 

 does at the ordinary temperature, kept up its activity for 

 at least an hour. For txample, in one experiment, the 

 electrodes being placed on the corneas so that the cur- 

 rents were balanced, sensibility was active for an hour 

 and a quarter, but half an hour later it had almost disap- 

 peared. In this experiment the sensibility of the eye is 

 shown by the large deflection produced by a single candle 

 at different distances, thus : — 



Distance of Candle Divisions of Galvanometer 



from Eye. Scale. 



9 feet 100 



6 feet l8o 



3 feet 230 



I foot 420 



Sensibility of the Optic Nerve. — When the retina is 

 entirely removed from the eye-ball, and the optic nerve is 



