n6 



NA TURE 



[Dec. 4, 1884 



Here are two tourmalines ; if you look through them toward 

 the light, you see the white light all round, i.e. they are trans- 

 parent. If I turn round one of these tourmalines the light is 

 extinguished, it is absolutely black, as though the tourmalines 

 were opaque. This is an illustration of what is called polarisa- 

 tion of light. I cannot speak to you about qualities of light 

 without speaking of the polarisation of light. I want to show 

 you a most beautiful effect of polarising light, before [Illustrating 

 a little further by means of this large mechanical illustration 

 which you have in the bowl of jelly. Now I put in the lantern 

 another instrument called a "Nicol prism." What you saw 

 first were two plates of the crystal tourmaline which came from 

 Brazil, I believe, having the property of letting light pass when 

 both plates are placed in one particular direction as regards their 

 axes of crystallisation, and extinguishing it when it passes through 

 the first plate held in another direction. We have now an instru- 

 ment which also gives rays of polarised light. A Nicol prism is 

 a piece of Iceland spar, cut in two and turned, one part relatively 

 to the other, in a very ingenious way, and put together again and 

 cemented into one by Canada balsam. The Nicol prism takes 

 advantage of the property which the spar has of double refrac- 

 tion, and produces the phenomenon which I now show you. 



I turn one prism round in a certain direction and you get 

 light, a maximum of light. I turn it through a right angle and 

 you get blackness. I turn it one quarter round again and get 

 maximum light ; one quarter more, maximum blackness ; one 

 quarter more and bright light. We rarely have such a grand 

 specimen of a Nicol prism as this. 



There is another way of producing polarised light. I stand 

 before that light and look at its reflection in a plate of glass on 

 the table through one of the Nicol prisms, which I turn round, 

 so. Now I must incline that piece of glass at a particular 

 angle, rather more than 45° ; I find a particular angle in which, 

 if I look at it and then turn the prism round in the hand, the 

 effect is absolutely to extinguish the light in one position and 

 to give it maximum brightness in another position. I use the 

 term "absolute" somewhat rashly. It is only a reduction to a 

 very small quantity of light, not an absolute annulment as we 

 have in the case of the two Nicol prisms used conjointly. Those 

 of you who have never heard of this before would not know 

 what I am talking about. As to the mechanics of the thing it 

 could only be explained to you by a course of lectures in physical 

 optics. The thing is this, vibrations of light must be in a definite 

 direction relatively to the line in which the light travels. 



Look at this diagram, the light goes from left to right ; we 

 have vibrations perpendicular to the line of transmission. There 

 is a line up and down which is the line of vibration. Imagine 

 here a source of light, violet light, and here in front of it is the 

 line of propagation. Sound vibrations are to and fro ; this is 

 transverse to the line of propagation. Here is another, perpen- 

 dicular to the diagram, still following the law of transverse 

 vibration ; here is another circular vibration. Imagine a long 

 rope, you whirl one end of it and you send a screw-like motion 

 running along ; you can get the circular motion in one direction 

 or in the opposite. 



Plane polarised light is light with the vibrations all in a single 

 plane, perpendicular to the plane through the ray which is 

 technically called the " plane of polarisation." Circular polarised 

 light consists of undulations of luminiferous ether having a circular 

 motion. Elliptically polarised light is something between the 

 two, not in a straight line, and not in a circular line ; the course 

 of vibration is an ellipse. Polarised light is light that performs 

 its motions continually in < me mode or direction. If in a straight 

 line it is plane polarised light; if in a circular direction it is 

 circularly polarised light ; when elliptical it is elliptically polarised 

 light. 



With Iceland spar, one unpolarised ray of light divides on 

 entering it into two rays of polarised light, by reason of it- 

 of double refraction, and the vibrations are perpendicular to one 

 another in the two emerging rays. Light is always polarised 

 when it is reflected from a plate of unsilvered glass, or water, at 

 a certain definite angle of 56° degrees for glass, or 52 for 

 water, the angle being reckoned in each case from a perpendicular 

 to the surface. The angle for water is the angle whose tangent 

 is 1 -4. I wish you to look at the polarisation with your own 

 eyes. Light from glass at 56° and from water at 52° goes 

 away vibrating perpendicularly to the plane of incidence "and 

 plane of reflection. 



_ We can distinguish it without the aid of an instrument. Time 

 is a phenomenon well known in physical optics as " Haidinger's 



Brushes." The discoverer is well known in Philadelphia as a 

 mineralogist, and the phenomenon I speak of goes by his 

 name. Look at the sky in a direction of 90° from the sun, 

 and you will see a yellow and blue cross, with the yellow toward 

 the sun, and from the sun, spreading out like two fox's tails with 

 blue between, and then two red brushes in the space at right 

 angles to the blue. If you do not see it, it is because your eyes 

 are not sensitive enough, but a little training will give them the 

 needed sensitiveness. 



If you cannot see it in this way try another method. Look 

 into a pail of water with a black bottom ; or take a clear glas>. 

 dish of water, rest it on a black cloth and look down at the 

 surface of the water on a day with a white cloudy sky (if there 

 is such a thing ever to be seen in Philadelphia). You will see 

 the white sky reflected in the basin of water at an angle of about 

 50'. Look at it with the head tipped to one side and 

 then again with the head tipped to the other side, keeping your 

 eyes on the water, and you will see Haidinger's brushes. Do not 

 do it fast or you will make yourself giddy. The explanation of 

 this is the refreshing of the sensibility of the retina. The 

 Haidinger's brush is always there, but you do not see it because 

 your eye is not sensitive enough. After once seeing it you always 

 see it ; it does not thrust itself inconveniently before you when 

 you do not want to see it. You can readily see it in a piece of 

 glass with dark cloth below it, or in a basin of water. 



I am going to conclude by telling you how we know the wave 

 lengths of light and how we know the frequency of the vibrations. 

 We shall actually make a measurement of the wave length of 

 the yellow light. I am going to show you the diffraction 

 spectrum. 



You see on the screen, 1 on each side of a central white bar of 

 light, a set of bars of light variegated colours, the first one, on 

 each side, showing blue or indigo colour about four inches from 

 the central white bar and red about four inches farther, with 

 vivid green between the blue and the red. That effect is produced 

 by a grating with 400 lines to the centimetre, engraved on glass, 

 which I now hold in my hand. The next grating has 3000 lines 

 on a Paris inch. You see the central space and on each side a 

 large number of spectra, blue at one end and red at the other. 

 The fact that, in the first spectrum red is about twice as far from 

 the centre as the blue, proves that a wave length of red light is 

 double that of blue light. 



I will now show you the operation of measuring the length of 

 a wave of sodium light, that is a light like that marked " D " 

 on the spectrum, a light produced by a spirit lamp with salt in 

 it. The sodium vapour is heated up to several thousand degrees, 

 when it becomes self luminous and gives such a light as we 

 get by throwing salt upon a spirit lamp in the game of snap- 

 dragon. 



I hold in my hand a beautiful grating of glass silvered by 

 I.iebig's process with metallic silver, a grating with 6480 lines 

 to the inch, belonging to my friend Prof. Barker, which he has 

 kindly brought here for us this evening. You will see the 

 brilliancy of colour as I turn the light reflected from the grating 

 toward you, and pass the beam round the room. You have now 

 seen directly with your own eyes these brilliant colours reflected 

 from the grating, and you have also seen them thrown upon the 

 screen from a grating placed in the lantern. With a grating of 

 17,000 lines — a much greater number of lines per inch than the 

 other — you will see how much further from the central bright 

 space the first spectrum is ; how much more this grating changes 

 the direction or diffraction of the beam of light. Here is the 

 centre of the grating, and there is the first spectrum You will 

 note that the violet light is least diffracted and the red light is 

 most diffracted. This diffraction of light first proved to us 

 definitely the reality of the undulatory theory of light. 



You ask why does not light go round a corner as sound does. 

 Light does go round a corner in these diffraction spectra ; it 

 is shown going round a corner, it passes through these bars and 

 is turned round an angle of 30". Light going round a corner 

 by instruments adapted to show the result, and to measure the 

 angles at which it is turned, is called the diffraction of light. 



1 can show- you an instrument which will measure the wave 

 lengths of li^ht. Without proving the formula, let me tell it to 

 you. A spirit lamp with salt sprinkled on the wick gives very 

 nearly homogeneous light, that is to say, light all of one wave 

 length, or all of the same period. I have a little grating which 

 I take in my hand. I look through this grating and see that 



ng the chromatic bands thrown upon the 



1 from a diffractii 



