Dec. 4, 1884] 



NA TURE 



117 



Kndle before me, Close behind it you see a blackened slip of 

 ■ood with two white marks on it ten inches asunder. The line 

 i>n which they are marked is placed perpendicular to the line at 

 which I shall go from it. When I look at this salted spirit lamp 

 1 see a series of spectra of yellow light. As I am somewhat 

 short-sighted I am making my eye see with this eyeglass and 

 the natural lenses of the eye what a long-sighted person would 

 {make out without an eyeglass. On that screen you saw a >uc- 

 Icession of spectra. I now look direct at the candle and what 

 ido I see ? I see a succession of five or six brilliantly coloured 

 spectra on each side of the car.dle. But when I look at the 

 salted spirit lamp, now I see ten spectra on one side and ten 

 on the other, each of which is a monochromatic band of light. 



I will measure the wave lengths of light thus. I walk away 

 to a considerable distance and look at the candle and mark-. I 

 See a set of spectra. The first white line is exactly behind 

 the candle. I want the first spectrum to the right of that white 

 line to fall exactly on the other white line, which is ten inches 

 from the first. As I walk away from it I see it is now very near it ; 

 it is now on it. Now the distance from my eye is to be measured, 

 and the problem is again to reduce feet to inches. The distance 

 from the spectrum of the flame to my eye is thirty-four feet nine 

 inches. Mr. President, how many inches is that? 417 inches, 

 in round numbers 420 inches. Then we have the proportion, 

 as 4J0 is to 10 so is the length from bar to bar of the grating to 

 the wave length of sodium light. That is to say, as forty-two is 

 to one. The distance from bar to bar is the four-hundredth 

 of a centimetre : therefore the forty-second part of the four- 

 ihundredth of a centimetre is the required wave length, or the 

 16, Sooth of a centimetre is the wave length according to our 

 simple, and easy, and hasty experiment. The true wave length 

 of sodium light, according to the most accurate measurement, 

 is about a 17,000th of a centimetre, which differs by scarcely 

 more than 1 per cent, from our result ! 



The only apparatus you see is this little grating ; it is a piece 

 of glass with four-tenths of an inch ruled with 400 fine lines. 

 Any of you who will take the trouble to buy one may measure 

 the wave lengths of a candle flame himself. I hope some of you 

 will be induced to make the experiment for yourselves. 



If I put salt on the flame of a spirit lamp, what do I see 

 through this grating ? I see merely a sharply defined yellow 

 light, constituting the_ spectrum of vaporised sodium, while from 

 the candle flame I see an exquisitely coloured spectrum, far 

 more beautiful than I showed you on the screen. I see in fact a 

 series ofspectrums on the two sides with the blue toward the 

 candle flame, and the red further out. I cannot get one definite- 

 thing to measure from in the spectrum from the candle flame as 

 I can with the flame of a spirit lamp with the salt thrown on it, 

 which gives, as I have said, a simple yellow light. The highest 

 blue light I see in the candle flame is now exactly on the 

 line. Now measure to my eye, it is forty-four feet four inches, 

 or 5 32 inches. The length of this wave then is the 532nd part 

 of the four-hundredth of a centimetre, which would be the 

 21,280th of a centimetre, say the 21,000th of a centimetre. Then 

 measure for the red, and you would find something like the 

 11,000th for the lowest of the red light. 



Lastly, how do we know the frequency of vibration ? 



Why, by the velocity of light. How do we 1 now that ? 

 We know it in a number of different ways, which I cannot 

 explain now, because time forbids. Take the velocity of light. 

 It is 187,000 British statute miles per second. But it is much 

 better to take a kilometre for the unit. That is about six-tenths 

 of a mile. The velocity is very accurately 300,000 kilometres 

 per second ; that is, 30,000,000,000 centimetres per second. 

 Take the wave length as the 17,000th of a centimetre, and you 

 find the frequency of the sodium light to be 510 million million 

 per second. There, then, you find a calculation of the fre- 



1 



Vibrating spherule embedded n an elastic solid. 



quency from a simple observation which you can all make for 

 yourselves. 



Lastly, I must tell you about the colour of the blue sky which 

 was illustrated by the spherule embedded in an elastic solid. I 

 want to explain to you in two minutes the mode of vibrations. 



Take the simplest plane polarised light. Here is a spherule 

 which is producing it in an elastic solid. Imagine the solid to 

 extend miles horizontally and miles down, and imagine this 

 spherule to vibrate up and down. It is quite clear that it will 

 make transverse vibrations similarly in all horizontal directions. 

 The plane of polarisation is defined as a plane perpendicular to 

 the line of vibration. Thus, light produced by a molecule 

 vibrating up and down, as this red globe in the jelly before 

 you, is polarised in a horizontal plane, because the vibrations 

 are vertical. 



Here is another mode of vibrations. Let me twist this 

 spherule in the jelly as I am doing it, and that will produce 

 vibrations, also spreading out equally in all horizontal directions. 

 When I twist this globe round, it draws the jelly round with it ; 

 twist it rapidly back, and the jelly flies back. By the inertia of 

 the jelly the vibrations spread in all directions, and the lines of 

 vibration are horizontal all through the jelly. Everywhere, 

 miles away, that solid is placed in vibration. You do not see it, 

 but you must understand that they are there. If it flies back it 

 makes vibration, and we have waves of horizontal vibrations 

 travelling out in all directions from the exciting molecule. 



I am now causing the red globe to vibrate to and fro horizon- 

 tally. That will cause vibrations to be produced which will be 

 parallel to the line of motion at all places of the plane perpen- 

 dicular to the range of the exciting molecule. What makes the 

 blue sky ? These are exactly the motions that make the blue 

 light of the sky, which is due to spherules in the luminiferous 

 ether, but little modified by the air. Think of the sun near the 

 horizon ; think of the light of the sun streaming through and 

 giving you the azure blue and violet overhead. Think first of 

 any one particle of the sun, and think of it moving in such a 

 way as to give horizontal and vertical vibrations and what not 

 of circular and elliptic vibrations. 



You see the blue sky in high-pressure steam blown into the 

 air ; you see it in the experiment of Tyndall's blue sky, in which 

 a delicate condensation of vapour gives rise to exactly the azure 

 blue of the sky. 



Now the motion of the luminiferous ether relatively to the 

 spherule gives rise to the same effect as would an opposite 

 motion impressed upon the spherule quite independently by an 

 independent force. So you may think of the blue colour coming 

 from the sky as being produced by to-and-fro vibrations of 

 matter in the air, which vibrates much as this little globe vibrates 

 embedded in the jelly. 



The result in a general way is this : The light coming from 

 the blue sky is polarised in a plane through the sun, but the 

 blue light of the sky is complicated by a great number of circum- 

 stances, and one of them is this, that the air is illuminated not 

 only by the sun but by the earth. If we could get the earth 

 covered by a black cloth, then we could study the polarised 

 light of the sky with simplicity, which we cannot do now. 

 There are, in Nature, reflections from seas and rocks and hills 

 and waters in an indefinitely complicated manner. 



Let observers observe the blue sky not only in winter when 

 the earth is covered with snow, but in summer when it is 

 covered with dark green foliage. This will help to unravel the 

 complicated phenomena in question. But the azure blue of the 

 sky is light produced by the reaction on the vibrating ether of 

 little spherules of water, of perhaps a fifty-thousandth or a hun- 

 dred-thousandth of a centimetre diameter, or perhaps little 

 motes, or lumps, or crystals of common salt, or particles of dust, 

 or germs of vegetable or animal species wafted about in the air. 

 Now what is the luminiferous ether? It is matter prodigioady 

 less dense than air — millions and millions and millions of times 

 less dense than air. We can form some sort of idea of its limita- 

 tions. We believe it is a real thing, with great rigidity ,n com- 

 parison with its density, and it may be made to vibrate 400 

 million million times per second and yet with such rigidity 

 as not to produce the slightest resistance to any body going 

 through it. 



Going back to the illustration of the shoemaker's wax : if a 

 cork will in the course of a year push its way up through a plate 

 of that waxwhen placed underwater, and if a lead bullet will pene- 

 trate downwards to the bottom, what is the law of the resistance ? 

 It clearly depends on time. The cork slowly in the course of a 

 year works its way up through two inches of that substance ; 

 give it one or two thousand years to do it and the resistance will 

 be enormously less ; thus the motion of a cork or bullet, at the 

 rate of one inch in 2000 years, may be compared with that of 

 the earth, moving at the rate of six times ninety-three million 

 miles a year, or nineteen miles per second, through the lumi- 



