August ii, 1892] 



NATURE 



343 



springs from chemical change in the muscular substance, and 

 therefore the muscle is more easily fatigued than the nerve. 

 The molecular motion in an excited nerve produces a momentary 

 electric current ; but that result is not peculiar to nerve. The 

 same occurs in muscle when stimulated. Possibly the mole- 

 cular movement is of the nature of a mechanical vibration ; 

 at all events, we know now that a nerve can transmit 

 hundreds, even thousands of impulses, or let us simply say 

 vibrations, per second. The fact is so important and 

 significant in relation to the physiology of the sense organs, that 

 I show you an experiment to render it more intelligible. A 

 frog's muscle has been hooked to a light lever to record its move- 

 ment on a smoked cylinder. The nerve of the muscle has been 

 laid on two electrodes connected with the secondary coil of an 

 induction machine. In the primary circuit a vibrating reed has 

 been introduced to serve as a key for making and breaking the 

 circuit, and so stimulating the nerve with periodic induction 

 shocks. If we make the reed long enough to vibrate ten 

 times per second, ten impulses are sent through the nerve to the 

 muscle and ten distinct contractions produced, as shown by the 

 wavy line upon the cylinder. If we shorten the reed so that it 

 will vibrate, say, fifty times per second, the muscle is thrown 

 into a continuous contraction and traces a smooth line on the 

 cylinder ; but if we listen to the muscle we can hear a tone 

 having a pitch of fifty vibrations per second, from which we 

 know that fifty nerve impulses are entering the muscle and in- 

 <lucing fifty shocks of chemical discharge in the muscular sub- 

 stance. If we take a reed that vibrates, say, 500 times per 

 second, we hear, on listening to the muscle, a tone having the 

 pitch of 500 vibrations. Observe, that we are not dealing with 

 the transmission of electrical shocks along the nerve, but with 

 the transmission of nerve impulses. By stimulating the nerve 

 with wires of a telephone it has been shown by D'Arsonval that 

 a nerve can transmit upwards of 5000 vibrations per second, and 

 that the wave-forms may be so perfect that the complex electri- 

 cal waves produced in the telephone by the vowel sounds can 

 be reproduced in the sound of a muscle after having been trans- 

 lated into nerve vibrations and transmitted along a nerve. Such 

 experiments go far in helping us towards a comprehension of 

 the capabilities of nerves in transmitting nerve vibrations of 

 great frequency and complicated wave form ; but although they 

 enable us reasonably to suppose that all the fibres of the auditory 

 nerve can transmit nerve vibrations, simple or complex, and 

 with a frequency similar to that of all audible tones, we en- 

 counter superlative difficulty in applying such a theory to the 

 sense of sight. In objective sound we have to deal with a com- 

 paratively simple wave motion, whose frequency of vibration is 

 not difficult to grasp even at the highest limit of audible sound 

 — about 40,000 vibrations per second. But in objective light 

 the frequency of vibration is so enormous — amounting to hundreds 

 of billions per second — that everyone feels the difficulty of form- 

 ing any conception of the manner in which different frequencies 

 of ether waves induce differences in colour sensation. 



But before passing to colour sense, I wish to allude for a 

 moment to the sense of smell. The terminals of the olfactory 

 nerve in the nose are epithelial cells. It has been recently 

 shown by Von Brunn ^ that in man and other mammals the cells 

 have at their free ends very delicate short hairs, resembling 

 those long known in lower vertebrates. These hairs must be 

 the terminal structures affected by substances that induce smell, 

 and are therefore analogous to the hairs on the terminal cells in 

 our organ of hearing. No one ever suggested that the hairs of 

 the auditory cells can analyze sounds by responding to par- 

 ticular vibrations, and I think it quite as improbable that the 

 hairs on any particular olfactory cell respond to the molecular 

 vibrations of any particular substance. If we follow those who 

 have had recourse to the doctrine of specific activities to explain 

 the production of different smells, we must suppose that at least 

 one special epithelial cell and nerve fibre are affected by each 

 different smelling substance. Considering how great is the 

 variety of smells, and that their number increases with the pro- 

 duction of new substances, it would be a somewhat serious 

 stretch of imagination to suppose that for each new smell of a 

 substance yet to emerge from the retort of the chemist there is 

 in waiting a special nerve terminal in the nose. It seems to me 

 far simpler to suppose that all the hairs of the olfactory cells are 

 affected by every smelling substance, and that the different 

 qualities of smell result from difference in the frequency and 

 form of the vibrations initiated by the action of the chemical 

 ■ Voxx'Brawa, Archivfiirinikrosko^itCf'ie Anatomic, 1892, Band 39. 



NO. T 189, VOL. 46] 



molecules on the olfactory cells and transmitted to the brain. 

 That hypothesis was, I believe, first suggested by Prof. Ramsay,* 

 of Bristol, in 1882, and it seems to me the only intelligible 

 theory of smell yet offered. But it must be admitted that a 

 theory of smell, such as that advanced by Ramsay, involves a 

 more subtle conception of the molecular vibrations in nerve 

 fibrils than is required in the case of hearing. It involves 

 the conception that musk, camphor, and similar substances pro- 

 duce their characteristic qualities of smell by setting up nerve 

 vibrations of different frequencies and probably of different com- 

 plexities. We shall see what bearing this may have on the 

 theory of colour sense, to which I now pass. 



No impressions derived from external Nature yield so much 

 calm joy to the mind as our sensation of colour. Pure tones 

 and perfect harmonies produce delightful sensations, but they 

 are outrivalled by the colour effects of a glorious sunset. With- 

 out our sense of colour all Nature would appear dressed in bold 

 black and white, or indifferent grey. We would recognize, as 

 now, the beauty of shapely forms, but they would be as the cold 

 engraving contrasted with the brilliant canvas of Titian. The 

 beautiful tints we so readily associate with natural objects are 

 all of them sensations produced in our brain. Paradox though 

 it appear, all Nature is really in darkness. The radiant energy 

 that streams from a sun is but a subtle wave-motion, which pro- 

 duces the common effects of heat on all bodies, dead or living. 

 It does not dispel the darkness of Nature until it falls on a living 

 eye, and produces the sense of light. Objective light is only a 

 wave motion in an ethereal medium ; subjective light is a 

 sensation produced by molecular vibration in our nerve appa- 

 ratus. 



The sensory mechanism concerned in sight consists of the 

 retina, the optic nerve, and the centre for visual sensation in 

 the occipital lobe of the brain. In the vertebrate eye the fibres 

 of the optic nerve spread out in the inner part of the retina, and 

 are connected with several layers of ganglionic cells placed ex- 

 ternal to them. The light has to stream through the fibres and 

 ganglionic layers to reach the visual cells — that i?, the nerve 

 terminals placed in the outer part of the retina. They may he 

 regarded as epithelial cells, whose peripheral ends are developed 

 into peculiar rod- and cone-shaped bodies, while their central 

 ends are in physiological continuity with nerve fibrils. Each 

 rod and cone consists of an inner and an outer segment. The 

 outer segment is a pile of exceedingly thin, transparent, doubly 

 refractive discs, colourless in the cone, but coloured pink or 

 purple in the rod. In man, the inner segment of both rod and 

 cone is colourless and transparent. Its outer part appears to be 

 a compact mass of fine fibrils that pass imperceptibly into the 

 homogeneous-looking protoplasm in the shaft of the cell. Owing 

 to the position of the rods and cones, the light first traverses 

 their inner, then their outer segments, and its unabsorbed portion 

 passes on to the adjacent layer of dark-brown pigment cells by 

 which it is absorbed. It is not necessary for me to discuss the 

 possible difference of function between the rods and cones. I 

 may simply say that in the central part of the yellow spot of the 

 retina, where vision is most acute, and from which we derive 

 most of our impressions of form and colour, the only sensory 

 terminals are the cones. A single cone can enable us to obtain 

 a distinct visual impression. If two small pencils of light fall 

 on the same cone the resulting sensory impression is single. To 

 produce a double impression the luminous pencils must fall on 

 at least two cones. That shows how distinct must be the path 

 pursued by the nerve impulse from a visual cell in the yellow 

 spot of the retina to a sensory cell in the tirain. The impulses 

 from adjacent terminals must pursue their own discrete paths 

 through the apparent labyrinth of nerve fibrils and ganglion 

 cells in the retina to the fibres of the optic nerve. How these 

 facts bear on the theory of colour sense will presently be ap- 

 parent. Meantime I pass to the physical agent that stinmlates 

 the retina. 



When a beam of white light is dispersed by a prism or dif- 

 fraction grating, the ether-waves are spread out in the order of 

 their frequency of undulation. The undulations of radiant 

 energy extend through a range of many octaves, but those able 

 to stimulate the retina are comprised within a range of rather 

 less than one octave, extending from a frequency of about 395 

 billions per second at the extreme red to about 757 billions at 

 the extreme violet end of the visible spectrum. The ultra-violet 

 waves in the spectrum of sunlight extend through rather more than 

 half an octave. Although mainly revealed by their chemical 

 ' Ramsay, Nature, 1882, vol. xxvi. p. 189. 



