Nov. 27, 1884] 



NATURE 



93 



I shall show the distinction between these vibrations and the 

 vibrations of light. Here is the fixed appearance of the particles 

 when displaced but not in motion. You can imagine particles 

 of something, the thing whose motion constitutes light. This 

 lliing we call the luminiferous ether. That is the only substance 

 we are confident of in dynamics. One thing we are sure of, and 

 that is the reality and substantiality of the luminiferous ether. 

 This instrument is merely a method of giving motion to a 

 designed for the purpose of illustrating wave motion of 

 light. I will show you the same thing in a fixed diagram, but 

 this arrangement shows the mode of motion. 



Now follow the motion of each particle. This represents a 

 f the luminiferous ether, moving at the greatest speed 

 when it is at the middle position. 



You see the two modes of vibration, 1 sound and light now 

 moving together, — the travelling of the wave of condensation 

 and rarefaction, and the travelling of the wave of transverse 

 displacement. Note the direction of propagation. Here it is 

 from your left to your right, as you look at it. Look at the motion 

 when made faster. We have now the direction reversed. The 

 propagation of the wave is from right to left, again the propaga- 

 tion of the wave is from left to right ; each particle moves per- 

 pendicularly to the line of propagation. 



I have given you an illustration of the vibration of sound 

 waves, but I must tell you that the movement illustrating the 

 condensation and rarefaction represented in that moving diagram 

 are necessarily very much exaggerated, to let the motion be per- 

 ceptible, whereas the greatest condensation in actual sound 

 motion is not more than one or two per cent, or a small fraction 

 of a per cent. Except that the amount of condensation was ex- 

 aggerated in the diagram for sound, you have a correct repre- 

 sentation of what actually takes place in the low note C. 



On the other hand, in the moving diagram representing light 

 waves what had we ? We had a great exaggeration of the incli- 



of Red Light. 



of Violet Light. 



nation of the line of particles. You must first imagine a line of 

 particles in a straight line, and then you must imagine them dis- 

 turbed in a wave curve, the shape of the curve corresponding to 

 the disturbance. Having seen what the propagation of the wave 

 is, look at this diagram and then look at that one. This, in 

 light, corresponds to the different sounds I spoke of at first. The 

 wave length of light is the distance from crest to crest of the wave, 

 or from hollow to hollow. I speak of crests and hollows, because 

 we have a diagram of ups and downs as the diagram is placed. 



1 li re, then, you have a wave length.- In this lower diagram 

 you have the wave length of violet light. It is but one-half the 

 length of the upper wave of red light ; the period of vibration 

 is but half as long. Now, on an enormous scale, exaggerated 

 not unly as to slope, but immensely magnified as to wave 

 length, we have an illustration of the waves of light. The 

 drawing marked "red'' corresponds to red light, and this 

 lower diagram corresponds to violet light. The upper curve 

 really corresponds to something a little below the red ray of 

 light in the spectrum, and the lower curve to something beyond 

 the violet light. The variation in length between the most ex- 

 treme rays is in the proportion of four and a half of red to eight 

 of the violet, instead of four and eight ; the red waves are nearly 

 as one to two of the violet. 



To make a comparison between the number of vibrations for 

 each wave of sound and the number of vibrations constituting 

 light waves, I may say that 30 vibrations per second is about the 

 smallest number which will produce a musical sound ; 50 per 

 second gives one of the grave pedal notes of an organ, 100 or 



1 Showing two moving diagrams, simultaneously, on the screen, one 

 depicting a wave nio<ion of light, the other a sound vibration. 



Exhibiting a large drawing, or chart, representing a red and a violet 

 wave of Ught. 



200 per second give the low notes of the bass voice, higher notes 

 with 250 per second, 300 per second, 1000, 40C0, up to 8000 

 per second give about the shrillest notes audible to the human 

 ear. 



Instead of the numbers, which we have, say in the most com- 

 monly used part of the musical scale, i.e. from 200 or 300 to 600 

 or 700 per second, we have millions and millions of vibrations 

 per second in light waves ; that is to say, 400 million million per 

 second, instead of 400 per second. That number of vibrations is 

 performed when we have red light produced. 



An exhibition of red light travelling through space from the 

 remotest star is due to the propagation by waves or vibrations, in 

 which each individual particle of the transmitting medium vibrates 

 to and fro 400 million million times in a second. 



Some people say they cannot understand a million million. 

 Those people cannot understand that twice two makes four. 

 That is the way I put it to people who talk to me about the 

 incomprehensibility of such large numbers. I say finitude is 

 incomprehensible, the infinite in the universe is comprehensible. 

 Now apply a little logic to this. Is the negation of infinitude 

 incomprehensible? What would you think of a universe in 

 which you could travel one, ten, or a thousand miles, or even to 

 California, and then find it come to an end ? Can you suppose 

 an end of matter, or an end of space ? The idea is incompre- 

 hensible. Even if you were to go millions and millions of miles 

 the idea of coming to an end is incomprehensible. 



You can understand one thousand per second as easily as you 

 can understand one per second. You can go from one to ten, 

 and ten times ten, and then to a thousand without taxing your 

 understanding, and then you can go on to a thousand million and 

 a million million. You can all understand it. 



Now 400 million million vibrations per second is the kind of 

 thing that exists as a factor in the illumination by red light. 

 Yiolet light, after what we have seen and have illustrated by that 

 curve, I need not tell you corresponds to vibrations of 800 

 million million per second. There are recognisable qualities of 

 light caused by vibrations of much greater frequency and much 

 less frequency than this. You may imagine vibrations having 

 about twice the frequency of violet light and one-fifteenth the 

 frequency of red light and still you do not pass the limit of the 

 range of continuous phenomena only a part of which constitutes 

 visible light. 



Everybody knows the " photographer's light " and has heard 

 of invisible light producing visible effects upon the chemically 

 prepared plate in the camera. Speaking in round numbers, I 

 may say that, in going up to about twice the frequency I have 

 mentioned for violet light, you have gone to the extreme end of 

 the range of known light of the highest rates of vibration ; I 

 mean to say that you have reached the greatest frequency that 

 has yet been observed. 



When you go below visible red light what have you ? We have 

 something we do not see with the eye, something that the 

 ordinary photographer does not bring out on his photographically 

 sensitive plates. It is light, but we do not see it. It is something 

 so closely continuous with light visible, that we may define it by 

 the name of invisible light. It is commonly called radiant heat ; 

 invisible radiant heat. Perhaps, in this thorny path of logic, 

 with hard words flying in our faces, the least troublesome way of 

 speaking of it is to call it radiant heat. The heat effect you 

 experience when you go near a bright, hot coal fire, or a hot 

 steam boiler ; or when you go near, but not over, a set of hot- 

 water pipes used for heating a house; the thing we perceive in 

 our face and hands when we go near a boiling pot and hold the 

 hand on a level with it, is radiant heat ; the heat of the hands 

 and face caused by a hot fire, or a hot kettle when held under 

 the kettle, is also radiant heat. 



You might readily make the experiment with an earthen tea- 

 pot ; it radiates heat better than polished silver. Hold your 

 hands below, and you perceive a sense of heat ; above the te.ipot 

 you get more heat ; either way you perceive heat. If held over 

 the teapot you readily understand that there is a little current of 

 air rising. If you put your hand under the teapot you get cold 

 air ; the upper side of your hand is heated by radiation, while 

 the lower side is fanned and is actually cooled by virtue of the 

 heated kettle above it. 



That perception by the sense of heat, is the perception of 

 something actually continuous with light. We have knowledge 

 of rays of radiant heat perceptible down to (in round numbers) 

 about four times the wave length, or one-fourth the period of 

 visible, or red light. Let us take red light at 400 million 



