Aiio'. 22, 1872] 



NATURE 



335 



Polarised light, as indicated at the oiit^tt, is distinguished from 

 common light by the presence of certain peculiarities not ordinarily 

 found, and tliese peculiarities are to be detected only by means 

 of special instruments. Liyht which has been reflected or trans- 

 milted at particular angles Irom various substances, light which 

 has been scattered by small particles, is found to be in tliis peculiar 

 condition. So likewise is light which has passed through this 

 transparent piece of Iceland spar, or Nicol's prism, as it is called. 

 Yet the light w hich has so passed through, and wliich is now 

 projected on the screen, is to the unaided eye in no way different 

 from the same light before its passage. Nevertheless, if we 

 e.\amine or analyse it by means of a second Nicol, we shall find 

 the peculiarity of its condition revealed. For if either of the 

 Nicols be turned f;radually round (and remember that ihey are 

 both transparent colourless blocks of ciystal) the light gradually 

 fades until, when it has been turned through a right angle, the 

 light is absolutely extinguished. On turning the Nicol further 

 the light revives, and afterwards again fades, in such a manner 

 that in a complete revolution the light is twice at its brightest, 

 and twice is extinguished. Now, light is due to extremely small 

 and rapid vibrations of a very subtle medium, which is supposed 

 to pervade all space. The fact that vibrations {i.e. motions to 

 and fro) in one direction can produce waves advancing in 

 another will be familiar to all of you who have watched the 

 movement of a cork floa'ing on the sea. You will have 

 noticed that the cork has simply moved up and down, or 

 nearly so, while the waves have passed, as it were, under it, 

 along the surface of the water. 



Now, in order to make clearer to our minds how this 

 wave motion is produced, I will throw the electric light 

 upon a machine devised for the purpose. You now see a 

 horizontal row of knobs As the slider is pushed in the 

 knobs at' one end begin to rise in succession until each has 

 in turn attained its greatest elevation. Immediately after 

 reaching its highest position it begins to descend ; so that 

 the knobs first rise and then fall in regular succession, and con- 

 tinue to rise and fall in the same manner so long as the motion 

 is continued. Each of the knobs, beginning from number one, 

 is thus successively at the highest position, while at the same 

 moment those immediately before and behind it are at lower 

 positions. And as the knob which is at the highest position 

 represents what we call the crest of the wave, the crest will pass 

 successively along all the knobs, beginning from the first. Thus 

 the waves are transmitted along the line, while the vibrations 

 take place across it. If the line of knobs represent the direction 

 of a ray, their motions will represent the vibrations and waves 

 to which the light is supposed to be due. In ordinary light 

 these vibrations ni.ay take place in any directions perpendicular 

 to the ray ; and the effect of the crystal of which the Nicol is 

 made, is to restrict these vibrations to a particular direction. In 

 the arrangement now before you the first Nicol causes the vibra- 

 tions to be altogether horizontal. When the second Nicol is 

 placed similarly to the fitst, it will obviously have no further 

 effect upon the light ; but if it be turned through an angle, it 

 will transmit only vibrations inclined to the horizontal at that 

 angle ; that is, only such part of the original horizontal vibra- 

 tions as can be brought into the inclined direction ; in other 

 words, it will transmit only part of the light. And as the incli- 

 nation is increased the part of the light transmitted will diminish, 

 until, when the second Nicol is in a position to transmit only 

 vertical vibrations (i.e., when it hcs turned through a right 

 angle), the light will v.anish. Such is an explanation of this 

 fundamental experiment in polarisation on the principle of 

 what is called the Wave Theory of Light ; and I have ventured 

 to give it in some detail, because it is the key to all others, and 

 forms a starting point for any who may desire to go further in 

 the subject ; and it is a remarkable feature in this Wave Theory 

 of Light that the results of many other experimental combina- 

 tions, to some of which we will now proceed, might be predicted 

 upon the principles already laid down. 



If a plate of crystal, such as selenite, be placed between the 

 two Nicols, and turned round in its o«n plane, it will be found 

 that in certain positions at right Dngles to one another no effect 

 is produced. These may be called neutral positions. In all 

 other positions the fieUl is tinted with colour, which is most 

 brilliant when the plate has been turned through half a r'ght 

 angle Irom a neutral position. If one of the Nicols be turned, 

 the selenite remaining still, the crjlour will fade and entirely 

 vanish when the Nicol has turned through half a right angle. 

 After this position the complementary colour will begin to 



appear, and will be brightest when the Nicol has completed a 

 right angle. 



The colours so produced depend upon the thickness of the 

 plate ; thus, if we take a plate of selenite merely split and not 

 ground to a uniform thickness, we shall have a variety of tints 

 indicating the thickness of each particular part ; or we may, by 

 a careful arrangement of suitable thicknesses, produced a coloured 

 pattern of delicacy and variety dependent only upon the skill 

 with which the pieces have been worked. 



A plate of the same crystal worked into a concave form is in- 

 teresting as showing not only that the colours are dependent 

 upon the thickness, but also that when, with an increasing or 

 diminishing thickness of crystal, they have run through their 

 cycle, they begin again ; in other words, that the phenomenon 

 is periodic. The field is then covered with a series of concentric 

 rings, each of which is tinted with colours in a regular order. 



In all these instances it is clear, from the experiments 

 themselves, as well as from other experiments which form no 

 part of our present subject, that the modifications which light 

 undergoes are due to the internal structure of the crystals used. 

 And it becomes a qtiestion of interest whether it be not possible, 

 by some mechanical process, performed upon a non-crystalline 

 substance, such as glass, so far to imitate a crystalline structure 

 as to reproduce some of the optical results already shown. For 

 this purpose let us take a bar of glass. On interposing it in its 

 natural state between the Nicols when crossed, we find that no 

 effect is produced in the dark field upon the screen. If, how- 

 ever, I merely press it as though with the intention of bending 

 or breaking it, there will be at once brought about a condition 

 of strain capable of affecting the vibrations of the ray falling 

 upon it, to such a degree that some of them will find their way 

 through the screen. And this result may be explained on pre- 

 cisely the same mechanical principles as in the case of the crystal. 

 The effect may be heightened by placing the piece of glass in a 

 vice, and screwing it up so as to bend or compress it to a 

 greater degree than was possible by the hanel alone. When this 

 is elone the direction and even the relative amount of torsion or 

 compression of the elifferent parts will be noteel down as it were 

 by the forms and hues of the figures thrown upon the screen. 



The same kind of effect is shown by a piece of glass unevenly 

 heated ; but better still by glass which has been rapidly anel 

 unevenly cooled, — unannealed glass, as it is called. In the 

 pieces now before you, the outside, having become first cooled 

 and solidified, has formed a rigid framework, to which all the 

 interior has been obliged to conform. The interior parts have 

 consequently undergone strains and pressures in different direc- 

 tions and in different elegrees, in accordance with which each 

 part has become the subject of a definite internal molecular 

 arrangement ; and these, by each in its own way, modifying 

 the light which they transmit, give rise to the figures now before 

 you. 



I will conclude this series of experiments by one which, 

 although not so beautiful or striking as those which you have 

 already seen, is still interesting as bringing the subject home to 

 us, and as the only application of polarisation to commercial life 

 which has yet been made. You will recollect the brilliant se- 

 cjuence of colour shown by a quaitz plate when submitted to 

 polarised light. Well, the effects produced by that quartz plate 

 are also produced by not only some other crystals, but, what is 

 very remarkable, also by many of their solutions, e.g. by that of 

 sugar. Into this tube I have put a solution of sugar ; when it 

 is placed before the lamp, polarisation colours are shown on the 

 screen, while the liquid itself remains colourless. If the solution 

 be strengthened by the addition of more sugar, the tints vary ; 

 and by accurate observation of the colours for different positions 

 of the Nicol, the strength of the solution may be determined. 

 An instrument constructed w ith proper means of registering these 

 phenomena with accuracy is called a saccharometer. 



These experiments might be multiplied almost indefinitely, 

 and many a long winter evening might be spent in following 

 polarisation into other branches of science upon which it has 

 something to say. For example, on examining a variety of vege- 

 table and animal tissues, slices of wood, fronds of fern, scales of 

 fish, hair, horn, mother of pearl, etc., with a suitable polariscope, 

 we shoulel find that they exhibit, internally, definite structural 

 characters, capable of affecting the light, which they transmit in 

 the same general way as do crystals. Or again, if we were to 

 apply the principles established in an early part of this lecture 

 to the conditions of sky, aspect, ard time of day under which the 

 photographer notices that he can obtain the tnost perfect image 



