POLARISATION 



279 



Franklin, Ross, Parry, M'Clure, M'Clintock, Hall, Nor- 

 denskiold, Greely, and other Arctic and Antarctic 

 explorers ; Barrow's Hvitory of Voyages to the Arctic 

 Reijiont ( 2 vols. 1818 and 1846 ) ; C. R. Markham's Thres- 

 hold of the Unknown Region ; A. R. Markham's Life of 

 Franklin ; itinburyh Review, April 1891 ; Journal and 

 Proceedings, Roy. Oeo. Soc. ; and the published narratives 

 of the various Arctic and Antarctic explorations. 



Polarisation. ( 1 ) Of Light. An ordinary 

 narrow beam of sunlight has no sides, and is always 

 divided into two equal beams by a crystal of Ice- 

 land spar; but if it has once been reflected from 

 class or water, it will then be found in general that 

 different results, as regards the intensities of these 

 two beams, are produced by turning the crystal of 

 Iceland spar round the axis of the beam into differ- 

 ent successive positions. The beam is no longer 

 the same all round, but has acquired sides. Un 

 the vibratory or undulatory theory of Light (q.v. ) 

 this shows that the vibrations must be transverse 

 to the direction of propagation (see POLARITY). 

 Suppose a long cord, fixed to a distant wall, to be 

 held in the hand ; apply a sharp up-and-down 

 movement ; an up-ana-down wave will run along 

 the cord to the wall ; this illustrates the mode of 

 vibration in a lieam of plane polarised light. Make 

 the hand move in a circle, in a direction contrary 

 to that of the hands of a clock ; a wave will run 

 along in the form of a screw ; this screw will have 

 the form, and will advance a*ter the fashion, 

 of a corkscrew ; this illustrates the mode of 

 vibration in a right-handed circularly polarised 

 beam of light. Make the hand move in a circle 

 clockwise ; the wave-screw advances in a left- 

 tiamled fashion ; this illustrates left-handed cir- 

 cularly polarised light. Make the hand move in 

 an ellipse; an elliptical disturbance travels, screw- 

 fashion, right- or left-handed a the case may be ; 

 this represents elliptically polarised light. Com- 

 municate a series of disturbances of the greatest 

 irregularity in which no one direction, up or down, 

 right or left, has on the whole any predominance ; 

 the irregular succession of transverse disturbances 

 which will travel along the cord will represent the 

 vibration in a lieam of common or natural light. 

 Assume that while communicating these irregular 

 disturbances the hand is hampered but not disabled 

 with reference to any particular direction, say up 

 and down ; the vibrations in that direction are on 

 the whole less than those from right to left ; and 

 the whole complex of irregular disturbances would, 

 if they wrote their own path, tend to fill up an 

 ellipse with their trace-marking rather than to fill 

 up a circle, as the vibrations in common light 

 would tend to do ; this would represent the nature 

 of the vibrations in partially polarised light. Now 

 suppose a slot in a board, winch will allow the cord 

 to swing from end to end of the slot, but will not 

 allow the cord to swing athwart the slot ; all those 

 oscillations or components of oscillation which are 

 parallel to the slot will be able to traverse the slot ; 

 bat those which are at right angles to these will 

 not be allowed to pass. On endeavouring to trans- 

 mit through the slot the complex of oscillations 

 which illustrate the vibrations of common or natural 

 light, it will be found that no motion at right 

 angles to the slot is transmitted, and that what 

 does pass through is a complex of irregular oscil- 

 lations restricted to the plane of the slot. A second 

 lot, at right angles to the first, will cut off the 

 whole of what passes through the first ; and the 

 propagation of transverse oscillations along a cord 

 may thus be entirely checked. If, however, the 

 second slot be parallel to the first, all the oscilla- 

 tions transmitted by the first may pass through it 

 also ; and if it lie in an intermediate direction, the 

 second slot will allow a proportion to pass, which 

 depends upon the angle between the two slots, 



being proportional to cos 9, where 9 is that angle. 

 The first slot illustrates the functions of a polar- 

 iser ; the second illustrates those of a second polar- 

 iser or analyser. A polariser reduces incident 

 common light to a plane polarised condition, and 

 an analyser at right angles to the polariser will 

 quench it altogether. 



The phenomena of polarised light were first 

 observed in sunlight reflected from water or glass. 

 Common or natural light so reflected is always, 

 except when it retraces its path by direct reflection, 

 more or less partially 'polarised by reflection. 1 

 The polarisation is more or less complete according 

 to the angle of incidence. At one particular angle 

 of incidence the reflected light is as nearly plane 

 polarised as the particular reflecting substance 

 employed can make it. At this angle, the so- 

 called ' angle of complete polarisation,' the reflected 

 and the refracted rays are ( or tend to be ) at right 

 angles to one another, and tan i = fi, where i is the 

 angle between the incident ray and the normal, 

 and n is the index of refraction (see REFRACTION). 

 Metal reflectors have no angle of complete, but 

 only of maximum, polarisation ; and even among 

 such substances as glass, which are usually said 

 to have an angle of complete polarisation by reflec- 

 tion, it is only those whose index of refraction = 

 1'46 which can completely polarise common light 

 by a single reflection. In tnat case the intensity 

 of the reflected plane polarised beam is to that of 

 the original incident beam of common light as 6*52 

 to 100, or 6 '52 per cent. The intensity of light 

 polarised by one reflection is therefore a good deal 

 less than the 50 per cent, which might be secured 

 by any contrivance which effectually acted in a 

 way analogous to the first slot above mentioned. 

 The intensity of light polarised by reflection is 

 greatly improved by using, instead of a single 

 reflecting plate, a pile of plates. A crystal of 

 tourmaline or of iodo-sulphate of quinine will, on 

 the whole, allow only light polarised in one par- 

 ticular plane to pass through ; but then it darkens 

 it and colours it. Advantage is accordingly taken 

 of the property of a doubly-refracting transparent 

 crystal, such as Iceland spar, of dividing an inci- 

 dent beam of common light into two equal beams, 

 which are, when they travel in principal sections of 

 the crystal, polarised in planes at right angles to 

 one another, and each of which possesses (absorp- 

 tion apart) half the intensity of the original 

 l>eam. See REFRACTION (DOUBLE). As these two 

 beams diverge from one another it is comparatively 

 easy to arrange that one of them shall remain 

 parallel to the axis of the incident beam and of the 

 apparatus, while the other is allowed to wander 

 away laterally : and this is the basis of the con- 

 struction of the prisms of Nicol, Foucault, Wollas- 

 ton, Rochon, and others, which receive incident 

 ordinary light and transmit plane polarised light. 



Two beams of plane polarised light can interfere 

 with one another (see INTERFERENCE) when their 

 vibrations are wholly or partly in the same direc- 

 tion, but not if they be at right angles to one 

 another ; and a beam of light polarised in any way 

 can give rise to the phenomena of Diffraction (q.v. ). 



On interposing in the path of a plane polarised 

 beam of light an analyser, so placed as to allow 

 none of that light to be transmitted, and then 

 placing in the course of the plane polarised beam 

 before it reaches the analyser a thin film of a 

 doubly-refracting substance, such as mica, the field 

 of view may become filled with light. The doubly- 

 refracting mm generally breaks the incident plane 

 polarised beam into two plane polarised beams, 

 which are, after emergence from the film, parallel 

 to one another and on the whole coincident if of 

 sufficient breadth. These two beams are differently 

 retarded in the mica ; and, according to the amount 



