THANSACTIONS OF THli SECTIONS. 141 



brain; audit i« mauifestly impossible, fts has been remarked by Hermann, "to 

 localize this relationship 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 occurring; in retinn, 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 con- 

 scious state occur — in retina, in optic nerve, or in brain ? The only method of an- 

 swering this question, 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 Holmgi-en in 

 Sweden, and by Professor Dewar and myself in this country, it can be done, so far 

 as the retina is concerned. In carrying out this method, Professor Dewar and I 

 found tliat light produced a change in the electrical condition of the retina in an 

 eye removed from the head or kept in normal conditions, and we ascertained that 

 the general phenomena of this change corresponded with our sensational experi- 

 ences of luminous impressions. AVe 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 phenomenon 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 

 foimd that it did so. The inference, therefore, is that the relation between degi'ee 

 or variation in stimulus and the corresponding sensation of a luminous impression 

 is a function of the sense-organ or retina. 



Mode of ina'estigatino the Sknsoiiy Organ it.sklf. 



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 tlie stimulus of light from revolving disks, 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 the 

 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 stimiili applied to the sense-organ separated from the brain 

 by some method the same as or analogous to ours. If in these circumstances the 

 sense-organ gives results similar to those observed in the phenomena of conscious- 

 ness, 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 re- 

 sidts are produced. Thus I have always held that, as there is little or no histolo- 

 gical evidence of complexity of structure in the retina capable of accounting for the 

 theory of Thomas Yomig 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 loolc to the complex structure of the corpora 

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

 planation of these facts. It may be objected that such scepticism simply removes 

 the difficulty a little further back ; but I think it better to search for facts than to 

 be content with an hypothesis. 



OONCLVSION, 



Time will ncit permit me to discuss other researches in this field of inquirj', nur 

 the interesting speculations which have sprung from them, biit I think I have said 

 enough to .show the line of advance in this direction. 



True it is thnt apparently the physiological causation ipf 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 



1870. 14 



