728 REPOBT— 1892. 



Section D.— BIOLOGY. 

 Pkesilent of the Sectiox — Professor W. EuiHEKFOiD, M.D., F.R.S., F.R.S.E. 



THURSDAY, AUGUST 4. 



The President delivered the following Address: — 



At the meeting of this Association held at Birmingham in 188G I had the 

 honour oF delivering a lecture on the sense of hearing, in which I criticised the 

 current theory of tone-sensation, and I propose on this occasion to discuss the 

 current theories regarding our sense of colour. 



I may premise that our conceptions of the outer world are entirely founded on 

 the experience gathered from our sensory impressions. Through our organs of 

 sensation, mechanical, chemical, and radiant energies impress our consciousness. 

 The manner in which the physical agents stimulate the peripheral sense-organs, 

 the nature of the movement transmitted through our nerves to the centres for 

 sensation in the hraiu, the manner in which different qualities of sensation are 

 there produced — all these are problems of endless interest to the physiologist and 

 psychologist. 



Every physiologist has acknowledged the profound significance of Johannes 

 Miiller's law of the specific energies — or, as we should rather say, the specific 

 activities of the sense-organs. To those unfamiliar with it I may explain it by 

 saying that if a motor nerve be stimulated, the obvious result is muscular move- 

 ment ; it matters not by what form of energy the nerve is stimulated — it may be 

 by electricity or heat, by a mechanical pinch or a chemical stimulus — the specific 

 result is muscular contraction. In like manner, when the nerve of sight is stimu- 

 lated — it may be by light falling on the retina, or by electricity, or mechanical 

 pressure, or by cutting the nerve — the invariable result is a lumiuous sensation, 

 because the impression is transmitted to cells in the centre for vision in the brain^ 

 whose specific function is to produce a sense of light. 



The same principle applies to the other sensory centres; when thrown into 

 activity, they each produce a special kind of sensation. The sun's rays falling on 

 the skin induce a sense of heat, but falling on the eye, they induce a sense of light. 

 In both cases, the physical agent is the same ; the difi'erence of result arises from 

 specific differences of function in the brain centres concerned in thermal and visual 

 sense. "We have no conception how it is that different kinds of sensation arise from 

 molecular movements in the different groujis of sensory cells ; we are as ignorant 

 of that as we are of the nature of consciousness itself. 



The subject I propose to discuss on this occasion is not the cause of the different 

 hinds of sensation proper to the different sense-organs, but the causes of some 

 qualities of sensation producible through one and the same sense-organ. 



The theory of tone-sensation proposed by Helmholtz is, that the ear contains an 

 elaborate series of nerve terminals capable of responding to tones varying in pitch 

 from .sixteen vibrations to upwards of 40,000 vibrations per second, and that at least 

 one different fibre in the auditory nerve, and at least one different cell in the centre 



