Sept. 28, 1876] 



NATURE 



485 



only one, two, and three times stronger. The importance of the 

 discovery of this remarkable law is, that it shows a distinct ma- 

 thematical relationship between stimulation and sensation. Pos- 

 sibly it may be found to have applications to other psychological 

 phenomena. May it not vary in different animals, and even in 

 different individuals ? 



Criticism of Fechner's Method. 

 It is quite noticeable, however, that in the case of each sense, 

 the law did not hold good throughout the whole range of varia- 

 tions in intensity of stimulus ; and it is not surprising, when we 

 consider the complexity of the conditions, that such should be 

 the case. All of these experiments were made in the case of 

 visual impressions, for example, on the living eye, connected by 

 the optic nerve with the brain ; and it is manifestly impossible, 

 as has been remarked by Hermann, "to localise this relation- 

 ship between sensational effect and variation in amount of 

 stimulus, which has been called the psycho-physical law of 

 Fechner." Between the sensational effect and the first contact 

 of the stimulus, there are a series of complicated processes occur- 

 ring in retina, nerve, and brain, processes undergoing incessant 

 modification by the interchanges between these tissues and the 

 warm circulating blood. In which of these does this relation 

 between stimulus and conscious state occur — in retina, in optic 

 nerve, or in brain ? The only method of answering this ques- 

 tion, so far as I know, is to examine the effects of stimulation 

 upon these parts separately. It is manifestly next to impossible 

 to do this in the case of the optic nerve and the brain ; but by 

 the method pursued by Holmgren, in Sweden, and by Prof. 

 Dewar and myself in this country, it can be done, so far as the 

 retina is concerned. In carrying out this method, Prof. Dewar 

 and I found that light produced a change in the electrical con- 

 dition of the retina in an eye removed from the head or kept in 

 normal conditions, and we ascertained that the general pheno- 

 mena of this change corresponded Vv'ith our sensational experi- 

 ences of luminous impressions. We were, therefore, entitled to 

 assume that the change in the electrical conditions of the retina, 

 produced by the action of light, might be regarded as a pheno- 

 menon intimately related to those changes in the brain which 

 result in consciousness of a luminous impression. Consequently 

 we had an opportunity of ascertaining whether or not Fechner's 

 law agreed with the effects of a stimulus of light in altering the 

 electrical condition of the retina, and we found that it did so. 

 The inference, therefore, is that the relation between degree or 

 variation in stimulus and the corresponding sensation of a 

 luminous impression, is a function of the sense organ or retina. 

 Mode of Investigating the Sensory Organ Itself. 



I may here remark that this mode of inquiring into sensory 

 impressions has by no means been exhausted. The subjective 

 method of observing sensational effect under the stimulus of light 

 from revolving discs, by the contrasting of colours, by comparison 

 of auditory sensations produced by tones of different intensity, 

 pitch, and quality, is always open to the charge that the results 

 may not be due to specific histological structure of the sense 

 organ, as is almost invariably assumed, but to structure of the 

 recipient of impressions from the sense organ, namely, the brain. 

 The only way of proving that the effects are due to structural 

 peculiarities of the sense organ is to examine the effects of 

 stimuli applied to the sense organ separated from the brain by 

 some method the same or analogous to ours. If in these cir- 

 cumstances the sense organ give results similar to those observed 

 m the phenomena of consciousness, then we may assume that 

 these results are due to specific peculiarities of the sense organ, 

 and not to the brain. If, on the other hand, the results do not 

 agree, then we must look in the brain for the mechanism by 

 which these different results are produced. Thus I have always 

 held, that as there is little or no histological evidence of com- 

 plexity of structure in the retina capable of accounting for the 

 theory of Thomas Young regarding the perception of colours, or 

 of the facts of colour-blindness, or of the sensibility of different 

 zones of the retina to lights of different colours, we may have to 

 look to the complex structure of the corpora quadrigemina, cere- 

 bellum, or some portion of the cerebral hemispheres for an ex- 

 planat ion of these facts. It may be objected that such scepti- 

 cism simply removes the difficulty a little further back, but I 

 think it is better to search for facts than to be contented with an 

 hypothesis. 



Conclusion. 



Time will not permit me to discuss other researches in this 

 field of inquiry, nor the interesting speculations which have 



sprung from them, but I think I have said enough to show the 

 line of advance in this direction. 



True it is that apparently the physiological causation of many 

 mental phenomena may be, in its precise nature, inaccessible to 

 direct proof, but it is our duty as physiologists to push legitimate 

 research as far as it will go. I would remark also that such 

 researches are not incompatible with those spiritual ideas, 

 matters of faith and not of science, which are the basis of our 

 most cherished hopes. They demand, however, caution in the 

 scrutiny of facts, and judgment in drawing conclusions from 

 them. More than in any other kind of scientific labour, perhaps, 

 it is of the utmost importance here to keep the mind unbiassed, 

 a task by no means easy. To maintain a calm unprejudiced 

 attitude to inquiries which seem to demand a change of opinion 

 regarding what was supposed to be final, requires an effort which 

 varies in different persons. Some find it comparatively easy to 

 do so, while others succeed only after a severe struggle. Still it 

 is the state of mind which a man true to science ought to aspire 

 to, so that while he will not be blown about by every wind of 

 doctrine, he may be ready to accept what is apparently true 

 when he has had it clearly put before him. 



In conclusion, let me observe that it would save not a little 

 heart-burning, and might possibly remove acrimony from various 

 scientific and social controversies, could we only remember that 

 it is not very probable that we, in this nineteenth century, have 

 arrived at the final solution of many problems which have puzzled 

 wise men from the earliest times. Probably we have got nearer 

 the truth, but it is presumptuous to suppose that we have reached 

 the ultimate truth. Many hypotheses much in favour at present 

 may turn out to be inadequate. Still if they serve as stepping 

 stones to something better, and to more rational conceptions of 

 the mysterious phenomena about us, they will have done good 

 service. In the meantime it is our duty vigorously to prosecute 

 research, in all departments, pushing ahead fearlessly, and with 

 that enthusiasm which is the prime mover in all great deeds, so 

 that we may be able to transmit our department of knowledge 

 to posterity not only less burdened with error, but with many 

 additions of truth. 



Prof. Turner, of Edinburgh, gave an account of his researches 

 into the structure of the placenta in mammals, and showed how 

 forms originally supposed to be distinct and unconnected by gra- 

 dations, were really but modifications of one fundamental type. 

 Thus the obstacle to the reception of the theory of evolution, 

 which had been supposed to be constituted by the various pla- 

 cental structures, did not exist. But it was difficult to see in 

 many respects what causes had determined the evolution. In 

 some cases it appeared that the great dilatation of blood capil- 

 laries in the uterus might be of advantage, because less force 

 would be required for the propulsion of the blood. Again, in 

 the upward ascent, there was complication of the placental struc- 

 tures with restriction of area ; and he supposed that with this 

 restriction there would be a diminished danger of haemorrhage 

 after parturition, and consequently greater safety. 



Mr. F. M. Balfour, Fellow of Trinity College, Cambridge, 

 read a paper On the DevdopTitent of the Prolovertebra: and 

 Muscle- plates in Elasmobranch Fishes. The most important 

 points on which he laid stress were the origin of the notochord 

 from the hypoblast, the splitting of the mesoblast from the hypo- 

 blast as two distinct lateral halves, the consequent appearance 

 of the body cavity at first as two cavities, the extension of the 

 body cavity on each side up to the summit of the muscle-plates, 

 and the derivation of a large portion of the voluntary muscular 

 system from the splanchnic or visceral layer of the mesoblast. 

 He compared these embryological facts with many occurring in 

 the Invertebrates, especially in Sagitta, in Brachiopods, and in 

 Echinoderms, showing how it was possible to unify them by 

 adopting Haeckel's gastraea theory, and by no other method. 

 Dr. Allen Thomson warmly commended Mr. Balfour's researches, 

 saying that it was quite a new thing for such a continuous series 

 of embryological papers of great importance to proceed from a 

 British investigator. 



Mr. G. J. Romanes, M.A., gave an account of his further 

 researches on the physiological functions of the Medusae this 

 summer. To this we shall return. 



Prof. Haeckel described two of the simplest forms of animals 

 with two Liyers in their body-wall — Haliphysema and Gastro- 

 physema. They were Coelelenterata of the simplest type ; the 

 first form had one body cavity ; in the second it was partly 

 divided into two cavities, whereof one was specially appropriated 

 to the formation of ova, the other to nutrition. If there had 



