Dec. 4, 1884] 



NA TURE 



^5 



have already referred to. A gloom was cast over the meeting 

 by the announcement of the sudden death of Prof. Kolbe, the 

 distinguished recipient of the Davy Medal. 



The Annual Dinner subsequently took place al Willis's 

 Rooms, the Treasurer being supported by the Lord Chancellor, 

 the Marquess of Salisbury, the Lord Mayor, and others. 



THE WAVE THEORY OF LIGHT 1 

 II. 

 ' 'unie our study of visible light, that is, undulations ex- 



tending from red to violet in the spectrum (which 1 am going 

 to show you presently), 1 would first point out on this chart that 

 in the section from letter "A " to letter "D" we h 

 effect and heating effect only ; but no ordinary chemical or 

 hie effect. Photographers can leave their usual sensi- 

 tive chemically prepared plates exposed to yellow light and red 

 light without experiencing any sensible effect ; but when you get 



toward the blue end of the spectrum the photographic effect 

 begins to tell, more and more as you get towards the violet end. 

 When you get beyond the violet, there is the invisible light 

 known chiefly by its chemical action. From yellow to violet we 

 have visual effect, heating effect, and chemical effect, all three ; 

 above the violet only chemical and heating effects, and so little 

 of the heating effect that it is scarcely perceptible. 



The prismatic spectrum is Newton's discovery of the compo- 

 sition of white light. White light consists of every variety of 

 colour from red to violet. Here, now, we have Newton's 

 prismatic spectrum produced by a prism. I will illustrate % 

 little in regard to the nature of colour by putting something 

 before the light which is like coloured glass ; it is coloured 

 gelatine. I will put in a plate of red gelatine which 1 

 prepared of chemical materials, and see what that will do. Of 

 nil the light passing to it from violet to red it only lets through 

 the ud and orange, giving a mixed reddish colour. 



Here is another plate of green gelatine. The green absorbs all 

 giving only green. Here is another plate absorbing 

 something from each portion of the spectrum, taking away a 

 great deal of the violet and giving a yellow or orange appear- 

 ance to the light. Here is another absorbing out the green, 

 leaving red, orange, and a very little faint green, and absorbing 

 out all the violet. 



W'hrn the spectium is very carefully produced, far more per- 

 fectly than Newton knew how to show it, we have a homo- 

 geneous spectrum. It must be noticed that Newton did not 

 understand what we call a homogeneous spectrum ; he did not 

 produce it, and does not point out in his writings the conditions 

 for producing it. With an exceedingly fine line of light we can 

 bring it out as in sunlight, like this upper picture, red, orange, 

 yellow, green, blue, indigo, and violet, according to Newton's 

 nomenclature. Newton never used a narrow beam of light, and 

 so could not have had a homogeneous spectrum. 



This is a diagram painted on glass and showing the colours as 

 we know them. It would take two or three hours if I were to 

 explain the subject of spectrum analysis to-night. We must 

 tear ourselves away from it. I will just read out to you the 

 wave lengths corresponding to the different positions in the sun's 

 spectrum of certain dark lines commonly called " Fraunhofer's 

 lines." I will take as a unit the one-hundred thou -andth of a 

 centimetre. A centimetre is -4 of an inch ; it is a rather small 

 half an inch. I take the thousandth of a centimetre and the 

 h of that as a unit. At the red end of the spectrum 

 the light in the neighbourhood of that black line "A " has for 

 its wave length 76 ; " B" has 6-87; "D" has 5'Sc. ; the 

 "frequency" for "A" is 3 - g times 100 million million; the 

 frequency of " D" light is 5-1 times 100 million million per 

 second. 



Now what force is concerned in those vibrations as compared 

 with sound at the rate of 400 vibrations per second ; suppose 

 for a moment the same matter was to move to and ! 



I I Hire delivered at the Academy of Music. Philadelphia, under the 

 auspices of the Franklin Institute, September 29, 1884, by Sir William 

 Thomson, F. R.S., LL. L). (Continued from p. 94). 



the same range, but 400 million million times per second. The 

 force required is as the square of the number expressing the 

 frequency. Double frequency would require quadruple force 

 for the vibration of the same body. Suppose I vibrate my hand 

 again, as I did before. If I move it once per second a moderate 

 force is required ; for it to vibrate ten times per second 100 times 

 as much force is required ; for 400 vibrations per second 160,000 

 times as much force. 



If I move my hand once per second through a space of a quarter 

 of an inch a very small force is required ; it would require very 

 considerable force to move it ten times a second, even through 

 so small a range ; but think of the force required to move a 

 tuning fork 400 times a second ; compare that with the force 

 required for a motion of 400 million million times a second. 

 If the mass moved is the same, and the range of motion is the 

 same, then the force would be one million million million 

 million times as great as the force required to move the prongs 

 of the tuning fork. It is as easy to understand that number as 

 any number like 2, 3, or 4. 



Consider gravely what that number means and what we are to 

 infer from it. What force is there in space between my eye and 

 that light ? What forces are there in spaci between our eyes 

 and the sun, and our eyes and the remotest visible star ? There 

 is matter and there is motion, but what magnitude of force may 

 there be ? 



I move through this " luminiferous ether" as if it were 

 nothing. But were there vibrations with such frequency in a 

 medium of steel or brass, they would be measured by millions 

 and millions and millions of tons action on a square inch of 

 matter. There are no such forces in our air. Comets make a 

 disturbance in the air, and perhaps the luminiferous ether is split 

 up by the motion of a comet through it. So when we explain 

 the nature of electricity, we explain it by a motion of the lumi- 

 niferous ether. We cannot say that it is electricity. What can 

 this luminiferous ether be? It is something that the planets 

 move through with the greatest ease. It permeates our air ; it 

 is nearly in the same condition, so far as our means of judging 

 are concerned, in our air and in the inter-planetary space. The 

 air disturbs it but little ; you may reduce air by air-pumps to the 

 hundred-thousandth of its density, and you make little effect 

 in the transmission of light through it. The luminiferous ether 

 is an elastic solid. The nearest analogy I can give you is this 

 jelly which you see. 1 The nearest analogy to the waves of light 

 is the motion, which you can imagine, of this elastic jelly, with 

 a ball of wood floating in the middle of it. Look there, when 

 with my hand I vibrate the little red ball up and down, or when 

 I turn it quickly round the vertical diameter, alternately in 

 opposite directions ; — that is the nearest representation I can 

 give you of the vibratiins of luminiferous ether. 



Another illustration is Scottish shoemaker's wax or Burgundy 

 pitch, but I know Scottish shoemaker's wax better. It is heavier 

 than water, and absolutely answers my purpose. I take a large 

 slab of the wax, place it in a glass jar filled with water, place a 

 number of corks on the lower side and bullets on the upper side. 

 It is brittle like the Trinidad pitch or Burgundy pitch which I 

 have in my hand. You can see how hard it is, but if left to 

 itself it flows like a fluid. The shoemaker's wax breaks with a 

 brittle fracture, but it is viscous and gradually yields. 



What we know of the luminiferous ether is that it has the 

 rigidity of a solid and gradually yields. W'hether or not it is 

 brittle and cracks we cannot yet tell, but I believe the discoveries 

 in electricity, and the motions of comets, and the marvellous spurts 

 of light from them, tend to show cracks in the luminiferous ether — 

 show a correspondence between the electric flash and the aurora 

 borealis and cracks in the luminiferous ether. Do not take this as 

 an assertion, it is hardly more than a vague scientific dream : but 

 you may regard the existence of the luminiferous ether as a 

 reality of science, that is, we have an all-pervading medium, an 

 elastic solid, with a great degree of rigidity ; its rigidity is so 

 prodigious in proportion to its density that the vibrations of light 

 in it have the frequencies I have mentioned, with the wave 

 lengths I have mentioned. 



The fundamental question as to whether or not luminiferous 

 ether has gravity has not been answered. We have no know- 

 ledge that the luminiferous ether is attracted by gravity ; it is 

 sometimes called imponderable because some people vainly 

 imagine that it has no weight. I call it matter with the same 

 kind of rigidity that this elastic jelly has. 



siting a large bowl of clear jelly 

 embedded in the surface near the centre. 



ith a small red 



