431- 



NA TURE 



{March 15, 1877 



say to the left, while if it fall on the other eye, the needle 

 swerves to the right. When the eye of the lobster, re- 

 moved from the body, was divided longitudinally into 

 segments, each segment was found sensitive to light. The 



Di<i>;raiu sh.iwiiig arrangeiucnt of apparatus in experiment oii living lobster 

 A, corneal surface, having electiode, E, in contact with it. B, portioji 

 of carapace removed so as to expose moist surface for elect. oae, e. 

 K, key. G, galvauomcter. An ows indicate direction of current. 



effect of light was then to increase the primary current? 

 but no inductive action was observed on withdrawal- 

 This observation is interesting as a confirmation of the 

 views of physiologists regarding the mode of action of a 

 conipound eye. 



Mode of Experiment on Eye of Fish. — An experiment 

 upon the e>e of a fish may be made in a very simple 

 way, by a method adopted in Prof. Strieker's labora- 

 tory in Vienna. Take a fish and give it a very small 

 dose of woorara. It soon becomes almost motionless, 

 and sinks in some cases to the bottom of the vesse'. 

 Tile animal would soon die in consequence of paralysis 

 of the movement of the gills necessary for respiration. 

 But, if we take the animal out of the water, put it upon a 

 glass plate, introduce a little bit of cork under each gill, 

 and then by means of an india-rubber tube placed in the 

 mouth, allow a little water to flow over the gills, the fish 

 will live out of water for many hours. By this method 

 may be made the experiment upon the eye of a fish with 

 like results. 



Obsei'vation on Human Eye. — Having succeeded in 

 detecting the action of light on the retma of the living 

 warm-blooded animal without any operative procedures, 

 It appeared possible to apply a similar method to the eye 

 of man. For this purpose, a small trough of clay or 

 paraffin was constructed lound the margin of the orbi% 

 bo as to contain a quantity of dilate salt solution, when 

 the body was placed horizontally and the head properly 

 secured. Into this solution the terminal of a non-polar- 

 isable electrode was introduced, and in order to complete 

 the circuit the other electrode was connected with a large 

 gutta-percha trough containing salt solution, into which 

 one of the hands was inserted. By a laborious process 

 of education it is possible to diminish largely the elec- 

 trical variation due to the involuntary movements of the 

 eye-ball, and by fixing the eye on one point with concen- 

 trated attention, another observer, watching the galvano- 

 meter, and altering the intensity of the light, can detect 

 an electrical variation similar to what is seen in other 

 animals. This method, however, is too exhausting and 

 uncertain to permit of quantitative observations being 

 made. 



Explanation of Variation in Direction of Current. — 

 One phenomenon particularly attracted the attention of 

 physiologists, and especially of those who first saw the 



experiments, viz., that sometimes, in the case of the eye 

 of the frog, light produced an increase in the electrical 

 current, and in other cases a diminution. This we could 

 not at first account for. But we have been able to make 

 out that the positive and negative variation, or the in- 

 crease or diminution of the natural current on the action 

 of light, depends upon the direction of the primary cur- 

 rent, when the cornea and brain are in circuit. If the 

 cornea be positive and the brain be negative, then light 

 produces an increase of the electrical current. If, on the 

 other hand, the cornea be negative and the brain positive, 

 light then produces a diminution in the electrical current. 

 It is thus eoncluhively shown that the current superadded, 

 or if we may use the language, induced by the action of 

 light, is always in the same direcdon ; only in the one 

 case it is added to, and in the other subtracted from, the 

 primary current. 



The Use of Eqtial and Opposite Currents. — Many 

 experiments were performed in which equal and opposite 

 currents were transmitted through the galvanometer at 

 the same time. By the use of resistance coils, it was not 

 difficult to balance the current from the eye ; but, owing 

 to the inconstancy of even a Daniell's cell in such expe- 

 riments as these, it was impossible to avoid fluctuations 

 which might possibly have been mistaken for those due 

 to the action of light. This difficulty was got over by 

 what was formerly called the double eye experiment, in 

 which two similar eyes are placed in reversed positions 

 on the electrodes, so that the current from the one neu- 

 tralises that of the other. When this is accomplished, it 

 is easy by means of a blackened box, having a shutter at 

 each side, to allow light to fall on either the one eye or 

 the other, and it is then shown that the galvanometer 

 needle moves either to the right or left, according to the 

 eye affected. Instead of removing the eyes from the head 

 and balancing them as just described, it is a much better 

 method to apply the two electrodes directly to the corneas 

 in their natural position. By a httle manipulation, it is 

 possible to obtain two positions that seemingly give no 

 electrical current. In these circums.ances, Hght, allowed 

 to fall on the one eye or the other, produces the effects 

 above detailed. 



Action of Polarised Light and Colours of Spectrum. — 

 The next point investigated was the action of polarised 

 light and the various complementary colours. Early 

 experiments, by passing light through solutions having 

 various absorptive powers and by the direct coloured rays 

 of the spectrum, &c., lead always to the same conclusion— 

 namely, that the most luminous rays produce the greatest 

 effect. For studying the action of polarised light, the 

 simple contrivance of a black box, having a hole on one 

 bide of it, placed over the eye, may be employed. Oppo- 

 site the hole two cylindrical tubes of brass, each carrying 

 a Nicol's prism, were placed, and between the two prisms 

 a thin plate of quartz is iniroduced, producing the various 

 colours of polarised light on rotating one of the prisms. 

 The general results were exactly the same as with the 

 colours of the spectrum. In all cases, the impact of the 

 yellow rays produced the greatest effect. It has also been 

 ascertained by this method that the effect of the impact of 

 light is much more regular than the effect of its removal. 

 The results of one series of observations are given in the 

 two following tables : — 



Action on Frog's Eye of Colours of Polarised Lit^ht. 



