451- 



NA TURE 



March 22, 1877 



still adherent to the sclerotic, no effect of light can be 

 detected. It now appeared possible to examine this 

 question by repeating Donders's experiment of focussing 

 an image on the optic disc in the uninjured eye, when no 

 electrical disturbance ought to occur. This was done in 

 the eye of the pigeon, but an image free from irradiation 

 on the optic disc could not be produced, and consequently 

 there was always an electrical effect observed. 



Exhaustion and Stimulation of the Retina. — When the 

 same light from a fixed position is allowed to act on the 

 eye for successive intervals of time, say two minutes of 

 light and two minutes of darkness, it gradually falls off 

 in electrical sensibility. Thus, a candle at nine inches 

 gives the following results when successively used as a 

 stimulus : — 



Initial Effect. Final Effect. 



These figures show a rapid fall of the initial effect. 

 In these circumstances, it is evident that the image being 



Diagram showing the recording portion of RegnauU's Chrorograph. A A, 

 limbs of recording fork, worked by electro-magnets, G G. c, stilette on 

 limb of recording tuning-fork, b b, levers in connection with armatures 

 of electro-magnets, F F, and bearing markers, dd, which, along with c, 

 record on e, a strip of blackened paper passing over pulley. 



always localised on the same minute portion of the retina, 

 only a few of the rods and cones of that structure are 

 really exhausted. If the eye be allowed repose in the 

 dark for a period of from half an hour to an hour, it will 

 regain as much as triple the exhausted sensibility. But 

 another mode of proving that only a minute portion of the 

 retina was affected was to show that an alteration of 

 position of the image by a slight movement of the lumi- 

 nous body was followed by a new electric variation. In 

 order to vary and extend the action of a retinal image it 

 is necessary to suspend a steady lamp by means of an 

 indiarubber cord or spiral spring, so as to be able, 

 by inducing vibrations in any direction, to stimulate in 

 rapid succession different retinal areas. On oscillating 

 a pendulum of this kind, an electrical variation is ob- 

 served whenever the amplitude of the vibrations is 

 increased, and by inducing a combination of vibrations, 

 the electrical variation observed corresponds to what 

 would be found if the luminous intensity were sixteen 

 times as great as that of the station; r/ lisht. Similar 



experiments may be made by throwing an image from a 

 small silver mirror connected with a metronome. The 

 rapid exhaustion of the eye may be most readily demon- 

 strated by cutting off the anterior half of the eye, leaving 

 the vitreous humour in contact with the retina, observing 

 the effect of a candle, and then subjecting it to the action 

 of a magnesium lamp. The sensibility will now be enor- 

 mously diminished. The electrical variations resulting 

 from the respective actions of a candle and a magnesium 

 lamp placed at the same distance from the eye were as 

 follows : — 



Initial Effect. Final Effect. 



Candle 38 78 



Magnes'um lamp ... 120 135 i , 



This experiment proves that an increase of 200 per cent 

 in the illuminating power of a source of light only triples 

 the electrical effect. Thus the eye becomes less sensitive 

 as the illumination increases. 



Chronometrical Observations. — \ series of experiments 

 have been begun with the object of measuiing the time re- 

 quired from the initial impact of light before electrical varia 

 tion is produced. As the electrical variation has been shown 

 to agree with our consciousness of luminous effects, it be- 

 came an interesting point to ascertain whether the time 

 occupied by the action of light upon the eye of the frog is 

 similar to the time occupied in its action upon the eye of 

 man. A good many years ago, Prof. Bonders and his 

 pupil, Schelske, performed a number of experiments by 

 which they determined that the time required by the 

 human being to observe light and to signal back the im- 

 pression occupied about j^^th of a second. That is to 

 say, -,\jth of a second is occupied by the action of light on 

 the eye, the transmission of nerve-current to the brain, the 

 change induced in the brain during perception and voli- 

 tion, the time for the transmission of the nerve-current to 

 the muscles, on signalling the result, and the time occu- 

 pied by muscular contraction. The true period of latent 

 stimulation in the case of man must therefore be a very 

 small fraction of a second. In order to attempt a solu- 

 tion of this problem a chronograph made by Dr. Kon'g, 

 of Paris, was employed. A diagram of the recording por- 

 tion of the instrument is given above. The experimental 

 arrangements were as follows : the galvanometer, the eye 

 apparatus, and the chronograph being in separate rooms, 

 one observer was stationed at the galvanometer for the 

 purpose of signalling the moment the needle worked, 

 which was recorded by one of the markers D in the 

 diagram, the other marker being used to register the time 

 of initial action. 



The Jirst experimetit was to transmit at a known 

 moment, through the eye circuit in the dark room, a 

 quantity of current equal in amount to the electrical 

 variation produced, when the eye was stimula.ted by a 

 flash of light from a vacuum tube, and to record the 

 difference of time between the origin of the current and 

 the observer's signal from the galvanometer. 



The second experiment was to flash a vacuum tube at a 

 known moment in a room where the eye was placed, 

 and to record as before the instant the galvanometer 

 was effected. From the first observation we ascertain 

 the minimum amount of time necessary to overcome the 

 inertia of the instrument, the observer's personal equa- 

 tion, and the signallmg under the conditions of the ex- 

 periment. If this result is subtracted from the record 

 of the second observation, the difference will repre- 

 sent the latent period of light stimulation. From a 

 large number of experiments made on the eye of the 

 frog we have found the latent period amounts to less than 

 ^'gth of a second, but its absolute value must be ascer- 

 tained by some method not liable to the variations that 

 are inevitable to the process described. Altogether the 

 problem is one of great difficulty, but further investigations] 

 are in progress. J. D, 



