464 



NA TURE 



{March 13, 1884 



reasoning upon them, in speaking of light and radiant heat ; but 

 if we distinctly define light as that which we consciously per- 

 ceive as light — v%ithout attempting to define consciousness, 

 because we cannot define consciousness any more than we 

 can define free will— we shall be safe. There is no question 

 that you see the thing ; if you see it, it is light. Well now, 

 when is radiant heat light? Radiant heat is light when its 

 frequency of vibration is between 400 million millions per second 

 and 800 million millions per second. When its frequency 

 is less than 400 million millions per second it is not light ; it i-; 

 invisible "infra-red " radiant heat. When its frequency is more 

 than 800 million millions per second, it is not light if we cannot 

 see it ; it is invisible ul'.ra-violet radiation, truly radiant heat, 

 but it is not so commonly called radiant heat because its heating 

 effect is known rather tlieoretically than by sensory perception, 

 or thermomelric or thermoscopic indications. Observations 

 which have been actually made by Langley and by Ahney on 

 radiant heat take us down about three octaves bel jw violet, and 

 we may hope 10 be brought considerably lower still by future 

 observation. We know at present in all about four octaves — 

 that is from one to two, two to four, four to eight, eight to .six- 

 teen, hundred million millions — of radiant heat. One octave of 

 radiant heat is perceptible to the eye as light, the octave from 

 400 million millions to 800 million millions. I borrow the word 

 octave from music, not in any mystic sense, nor as indicating any 

 relation between harmony of colours and harmony of sound. 

 No relation exists between harmony of sound and harmony of 

 colours. I merely use the w>rd "octave" as a brief expres- 

 sion for any range of frequencies lying H'ithin the ratio of one 

 to two. If you double the frequency of a musical note, you 

 raise it an octave : in that sense I use the word for the moment 

 in respect to light, and in no other sense. Well now, 

 think what a tremendous chasm there is between the 100 million 

 millions per second, which is about the gravest hitherto disco- 

 vered note of invisible radiant heat, and the 10,000 per second, 

 the greatest number of vibrations in sound. This is an unknown 

 province of science : the investigation of vibrations between those 

 two limits is perhaps one of the most promising provinces of 

 science for the future investigator. 



In conclusion, I wish to bring before you the idea that all the 

 senses are related to force. The sense of sound, we have seen, 

 is merely a sense of very rapid changes of air pressUie (which is 

 force) on the drum of the ear. I have passed merely by name 

 over the senses of ta-te and smell. I may say they are chemical 

 senses. Taste common salt and taste su.;ar — you tell in a 

 moment the difference. The percep ion of that difference is a 

 perception of chemical quality. Well, there is a subtle molecu- 

 lar influence here, due to the touch of the object, on the tongue 

 or the palate, and producing a sensation which is a very different 

 thing from the ordinarily reckoned sense of touch, in the case 

 now considered, telling only of roughness and of temperature. 

 The most subtle of our senses perhaps is sight ; next come smell 

 and taste. Prof. .Stokes recently told me that he would rather 

 look upon taste and smell and sight as being continuous because 

 they are all molecular — they all deal with properties of matter, 

 not in the gross, but molecular actions of matter ; he would 

 rather group those three together than he would couple any of 

 them with any of the other senses. It is not necessary, however, 

 for us to reduce all the six senses to one, but I would just point 

 out that they are all related to force. Chemical action is a force, 

 tearing molecules apart, throwing or pushing them together : and 

 <mr chemical sense or senses may therefore sd far at le:ist be 

 regarded as concerned \\\\.\\ force. That the senses of smell and 

 taste are related to one another seems obvious ; and if physio- 

 logists v\ould pardon me, I would suggest that they may, with- 

 out impropriety, be regar.ied as extremes of one sense. This at 

 .all events can be said of them, they can be compared — « hich 

 cannot be said of any other two senses. You cannot say that 

 the shape of a cube, or the roughness of a piece of loaf sugar or 

 sandstone, is comparable with the temperature of hot water, or 

 is like the sound of a trumpet, or that the sound of a trumpet is 

 like scarlet, or like a rocket, or like a blue-light signal. There 

 is no comparability between any of these perceptions. Hut if 

 any one says, "That piece of cinnamon tastes like its smell," I 

 think he will express srjmething of general experience. The 

 smell and the taste of pepper, nutmeg, cloves, cinnamon, vanilla, 

 apples, strawberries, and other articles of food, particularly 

 spices and fruits, have very marked qualities, in which the taste 

 and the smell seem essentially comparable. It does seem to me, 

 although anatomists distinguish between them, because the 



sensory organs concerned are different and because they have 

 not discovered a continuity between these organs, vve should not 

 be philosophically wrong in saying that smell and ta^te are 

 extremes of one sense — one kind of perceptivity — a sense of 

 chemical quality materially presented to us. 



Now sense of light and sense of heat are very different ; but 

 we cannot define the difference. \ou perceive the heat of a hot 

 kettle — how ? By its radiant heat against the face — ^that is one 

 way. But there is another way, not by radiant heat, of which 

 I shall speak later. You perceive by vision, but still in virtue 

 of radiant heat, a hot body, if illuminated by light, or if hot 

 enough to be sslf-luminous, red-hot or white-hot, you see it ; 

 you can both see a hot body, and perceive it by its heat, other- 

 wise than by seeing it. Take a piece of red hot cinder with the 

 tongs, or a red-hot poker, and study it ; carry it into a dark 

 room, and look at it. You see it for a certain time ; after a 

 certain time you cease to see it, but you still perceive radiant heat 

 from it. Well now there is radiant heat perceived by the eye 

 and the face and the hands all the time ; but it is perceived only 

 by the sense of temperature, when the hot body ceases to be red- 

 hot. There is then, to our senses, an absolute distinction in 

 modes of perception b-itween that which is continuous in the 

 external nature of the thing, namely, radiant heat in its vi-ible 

 and invisible varieties. It operates upon our senses in a way 

 that I cannot ask anatomists to admit to be one and the same in 

 both cases. They cannot now, at all events, say that there is an 

 alisolute continuity between the retina of the eye in its perception 

 of radiant heat as light, and the skin of the hand in its perception 

 of radiant heat as heat. We may come to know more; it may 

 yet appear that there is a continuity. Soneof Darwin's sublime 

 specula'ions may become realities to us ; and we may come to 

 recognise a cultivable retina all over the body. We have not 

 done that yet, but Darwin's grand idea occurs as suggesting that 

 there may be an absolute continuity betw -en the perception of 

 radiant heat by the retina of the eye and its perception by the 

 tissues and nerves concerned in the mere sense of heat. We 

 must be c mtent in the meantime, however, to make a distinction 

 between the senses of light a id heat. And indeed it must be 

 remarked that our sense of heat is not excited by radiant 

 heat only, while it is only and essentially radiant heat that 

 gives to the retina the sense of light. H Id your hand 

 under a red-hot poker in a dark roam; y ju perceive it to 

 be hot solely by its radiant heat, and you see it also by its 

 radiant heat. Now place the hand over it : you feel more of 

 heat. Now, in fact, you perceive its heat in three w lys — by 

 contact with the lieated air which has ascended from the poker, 

 and by radiant heat felt by your sense of heat, and by radiant 

 heal seen as light (the iri)n being still red-hot). But the sense of 

 heat is the same throughout, and is a certain effect experienced 

 by the tissue, whether it be caused by radiant heat, or by contact 

 with heated particles of the air. 



Lastly, there remains — and I am afraid I have already taxed 

 your patience too long — the sense of force. I have been vehe- 

 mently atacked for asserting this sixth sense. I need not go into 

 the controversy ; I need not explain to you the ground on which 

 I have been attacked ; I could not in fact, because in reading the 

 attack I have not been able to understand it myself. The only 

 tangible ground of attack, perhaps, was that a writer in New 

 York published this theory in iSSo. I had quoted Dr. Thomas 

 Reid, without giving a d.ate ; his date chances to be 1780 or 

 thereabouts. But physio'ogisis have very strenuously resisted 

 admitting that the sense of roughness is the same as that muscular 

 sense which the metaphysicians who followed Dr. Thomas Reid 

 in the University of Glasgow, taught. It was in the University 

 of Glasgow that I learnel the muscular sense, and I have not 

 seen it very distinctly stated elsewhere. What is this " muscular 

 sense" ? I press upon the desk before me with my right hand, 

 or I walk forward holding out my hand in the dark, and using 

 this means to feel my way, as a blind man does constantly who 

 finds wdiere he is, and guides himself, by the sense of touch. 

 I walk on until I perceive an obstruction by a sense of force 

 in the palm of the hand. How and where do I perceive 

 this sensation? Anatomists will tell you it is felt in the 

 muscles of the arm. Here, then, is a force which I perceive in 

 the muscles of the arm, and the corresponding perceptivity is 

 properly enough called a muscular sen e. But now take the tip 

 of your finger and rnb a piece of sandstone, or a piece of loaf 

 sugar, or a sm 10th fable. Take a inece of loaf sugar between 

 your finger and thumb, and take a smooth glass between your 

 finger and thumb. You i)erceive a difference. What is that 



