468 



NATURE 



[September 14, 1893 



between it and the normal function. It may help to illustrate 

 this relation to refer to the expressive word Atislosung by which 

 it has for mmy years been designated by German writers. This 

 word stands for the performance of function by the "letting off" 

 of "specific energies." Carrying out the notion of "letting 

 off" as expressing the link between action and reaction, we 

 might compare the whole process to the mode of working of a 

 repeating clock (or other similar mechanism), in which case 

 the pressure of the finger on the button would represent the ex- 

 ternal influence or stimulus, the striking of the clock, the normal 

 reaction. And now may I ask you to consider in detail one or 

 two illustrations of physiological reaction — of the letting off of 

 specific energy ? 



The repeater may serve as a good example, inasmuch as it is, 

 in biological language, a highly differentiated structure, to 

 which a single function is assigned. So also in the living 

 organism, we find the best examples of specific energy where 

 MiiUer found them, namely, in the most differentiated, or, as 

 weaieapt to call them, the highest structures. The retina, 

 with the part of the brain which belongs to it, together consti- 

 tute such a structure, and will afford us therefore the illustration 

 we want, with this advantage for our present purpose, that the 

 phenomena are such as we all have it in our power to observe 

 in ourselves. In the visual apparatus the p; inciple of normality 

 of reaction is fully exemplified. In the physical sense the word 

 "light" stands for ether vibrations, but in the sensuous or 

 subjective sense for sensations. The swings are the stimulus, 

 the sensations are the leiction. Between the two comes the 

 link, the "letting off," which it is our business to understand. 

 Here let us remember that the man who first recognised this 

 distinction between the physical and the physiological was not a 

 biologist, but a physicist. It was Young who first made clear 

 (though his doctrine fell on unappreciativc ears) that, although in 

 vision the external influences which give rise to the sensation of 

 light are infinitely varied, the responses need not be more than 

 three in number, each being, in MUUer's language, a "specific 

 energy " of some part of the visual apparatus. We speak of 

 the organ of vision as highly differentiated, an expression « hich 

 carries with it the suggesti< n of a distinction of rank between 

 different vital processes. The suggestion is a true one ; for it 

 would be poi-sible to arrange all those parts or organs of which 

 the bodies of the highet animals consist in a series, placing at 

 the lower end of the series those of which the functions are con- 

 tinuous, and therefore cilled vegetative ; at the other, those 

 highly specialised structures, a=, e.g., those in the brain, which 

 in response to physical light produce physiological, that is sub- 

 jective, light ; or, to take another instance, the so-called motor 

 cells of the surface of the brain, which in response to a stimulus 

 of much greater complexity produce voluntary motion. And 

 just as in civilised society an individual is valued according to 

 his power of doing one thing well, so the high rank which is 

 assigned to the structure, or rather to the "specific energy" 

 which it represents, belongs to it by virtue of its specialisation. 

 And if it be asked h iw this conformity is manifested, the 

 answer is, by the quality, intensity, duration, and extension 

 of the response, in all which respects vision serves as so good 

 an example, that we can readily understand how it happened 

 that it was in this field that the relation between response and 

 stimulus was first clearly recognised. I need scarcely say that, 

 however interesting it might be to follow out the lines of inquiry 

 thus indicated, we cannot attempt it this evening. All that I 

 can do is to mention oie or two recent observations which, 

 while they serve as illustrations, may perhaps be sufficiently 

 novel to interest even those who are at home in the subject. 



Probably every one is acquainted with some of the familiar 

 proofs that an object is seen for a much longer period than it is 

 actually exposed to view ; that the visual reaction lasts much 

 longer than its cause. More precise observations teach us that 

 this response is regulated according to laws which it has in com- 

 mon with all the higher functions of an organism. If, for 

 example, the cells in the brain of the torpedo are "let off"— - 

 that is, awakened by an external stimulus — the electrical dis- 

 charge, which, as in the case of vision, follows after a certain 

 interval, lasts a certain time, first rapidly increasing to a maxi- 

 mum of intensity, then more slowly diminishing. In like 

 manner, as regards the visual apparatus, we have, in the 

 response to a sudden invasion of the eye by light, a rise and fall 

 of a similar character. In the case of the electrical organ, and 

 in many analogous instances, it is easy to investigate the time 

 relations of the successive phenomena, so as to represent them 



graphically. Again, it is found that in many physiological re- 

 actions, the period of rising " energy" (as Ilelmholtz called it) 

 is followed liy a period during which the responding structure 

 is not only inactive, but its capacity for energising is so com- 

 pletely lost that the same exciting cause which a moment before 

 "let off" the characteristic response is now without effect. Aj 

 regards vision, it has long been believed that these general 

 characteristics of physiological reaction have their counterpart 

 in the visual process, the most striking evidence being that in 

 the contemplation of a lightning flash — or better, of an instan- 

 taneously illuminated white disc' — the eye seems to receive a 

 double stroke, indicating that, although the stimulus is single 

 and instantaneous, the response is reduplicated. The most pre- 

 cise of the methods we until lately possessed for investigating 

 the wax and wane of the visual reaction, were not only diflScult 

 to carry out but left a large margin of uncertainty. It was 

 therefore particularly satisfactory when M. Charpentier, of 

 Nancy, whose merits as an investigator are perhaps less known 

 than they deserve to be, devised an experiment of extreme .'im- 

 plicily which enables "us, not only to observe, but to measure 

 with great facility both phases of the reaction. It is difficult to 

 explain even the simplest apparatus without diagrams ; you will, 

 however, understand the experiment if you will imagine that you 

 are contemplating a disc, like those ordinarily used for colour 

 mixing ; that it is divided by two radial lines which diverge 

 from each other at an angle of 60° ; that the sector whici these 

 lines enclose is white, the rest black ; that the disc revolves 

 slowly, about once in two seconds. You thtn see, close to the 

 front edge of the advancing sector, a black bar, followed bv a 

 second at the same distance from itself but much fainter. Now 

 the scientific value of the experiment consists in this, that the 

 angular distance of the bar from the black border is in ])ro- 

 portion to the frequency of the revolutions — the faster the wider. 

 If, for example, when the disc makes half a revolution in a 

 second the distance is ten degrees, this obviously means that 

 when light bursts into the eye, the extinction happens one- 

 eighteenth of a second after the excitation. - 



The fact thus demonstrated, that the visual reaction conse- 

 quent on an instantaneous illumination- exhibits the alternations 

 I have described, has enabled M. Charpentier to make out 

 another fact in relation to the visual reaction which is, I think, 

 of equal importance. In all the instances, excepting the retina, 

 in which the physiological response to stimulus has a ilefinite 

 time-limitation, and in so far resembles an explosion — in other 

 words, in all the higher forms of specific energy, it can be; shown 

 experimentally that the process is propagated from the part first 

 directly acted on to other contiguous parts of similar endow- 

 ment. Thus in the simplest of all known phenomena of this 

 kind, the electrical change, by which the leaf of the Dionsea 

 plant responds to the slightest touch of its sensitive hairs, is 

 propagated from one side of the leaf to the other, so that in the 

 opposite lobe the response occurs after a delay which is pro- 

 portional to the distance between the spot excited and the spot 

 observed. That in the retina there is also such propagation has 

 not only been surmised from analogy, but inferred from certain 

 observed facts. M. Charpentier has now been able by a method 

 which, although simple, I must not attempt to describe, rot only 

 to prove its existence, but to measure its rate of progress over 

 the visual field. 



There is another aspect of the visual response to the slimums 

 of light which, if I am not trespassing too long on your 

 patience, may, I think, be interesting to consider. As the re- 

 lations between the sensations of colour and the physical 

 properties of the light which excites them, are among the most 

 certain and invariable in the whole range of vital reactions, it is 

 obvious that they afford as fruitful a field for physiological in- 

 vestigation as those in which white light is concerned. We have 

 on one side physical facts, that is, wave-lengths or vibration- 

 rates ; on the other, facts in consciousness— namely, sensations 

 of colour -so simple that notwithstanding their subjective 

 character there is no difficulty in measuring either their intensity 

 or their duration. Between these there are lines of infinenct, 

 nei'her physical nor psychological, which pass from the foimer 

 to the latter through the visual apparatus (retina, nerve, brain). 



1 The phenomenon is best seen when, in a dark room, the I'gl". "f ,* 

 luminous spark is thrown on to a white screen with the aid of a suiUWe 

 lens. J .» 



- Charpentier, " Riaclion oscillaloirc de la Ritine sous 1 influence ^s »" 

 citati ins lumineuses," Archives de I'liysiol., vol. xxiv. p. 541, and Profa- 

 gation de faction oscittatoire, &c., p. 362. 



NO. 1246, VOL. 48] 



