324 



NATURE 



{Feb. 26, 1874 



at an angle of 45° to that of reflexion (that of the paper 

 in the figure) the amplitudes of the two vibrations will be 

 equal ; and all the conditions will be fulfilled for the 

 production of circular polarisation. Such an instrument 

 was invented by Fresnel, and is called in consequence 

 Fresnel's rhomb. On account of its length and its dis- 

 placement of the ray, it is not so convenient as a quarter- 

 undulation plate ; but on the other hand it affects rays 

 of all wave-lengths equally, while the quarter-undulation 

 plate can strictly be adapted to rays of only one wave- 

 length. 



If either of these instruments be introduced and suit- 

 ably placed between a selcnite plate and the analyser, the 

 chromatic effects will be similar to those due to a plate of 

 quartz cut perpendicularly to the axis. 



Another important property of these instruments con- 

 sists in their effect upon circularly polarised light. Such 

 light may be considered to arise from two plane-polarised 

 rays whose vibrations are perpendicular to one another, 

 and which present a difference of phase equal to a quarter 

 of a wave-length. If, therefore, either a quarter-undu- 

 lation plate or a Fresnel's rhomb be suitably placed, it 

 will either increase or diminish the difference of phase by 

 a quarter of a wave-length. In the one case the differ- 

 ence of phase will amount to a half wave-length, in the 

 other it will vanish. And in either case the vibration will 

 be converted into a rectiimear one ; but the directions of 

 vibration in the two cases will be perpendicular to one 

 another. 



Reflexion from a metallic surface may also be employed 

 for converting plane into circular polarisation. If a ray 

 of plane-polarised light fall upon a metallic reflector it is 

 divided into two, whose vibrations are respectively paral- 

 lel and perpendicular to the reflector ; and the latter is 

 retarded behind the former by a difference of phase 

 depending upon the angle of incidence. If the plane of 

 vibration of the incident ray be inclined to the plane of 

 incidence at an angle (nearly 45°) which varies with the 

 metal employed, but which is perfectly definite, the inten- 

 sities become equal. And further, if the angle of inci- 

 dence have a particular value dependent upon the nature 

 of the metal (for silver 72") the retardation will amount 

 to a quarter of a wave-length. And the result will be a 

 circularly polarised ray as in the case of total reflexion. 



The apparatus (Fig. 15) best adapted for experiments 

 based upon this principle is a modification of Norremberg's 

 polariscope, srrggesled by Sir Charles Wheatsone, from 

 whom the following description is quoted : — 



"A plate of black glass, G, is fixed at an angle of 3" to 

 the horizon. The fihn to be examined is to be placed on 

 a diaphram, D, so that the light reflected at the polarising- 

 angle from the glass plate shall pass through it at right 

 angles, and, after reflexion at an angle of 18" from the 

 surface of a polished silver plate S, shall proceed ver- 

 tically upwards. N is a Nicol's prism, or any other 

 analyser, placed in the path of the second reflexion. 

 The diaphragm is furnished with a ring, moveable in its 

 own plane, by which the crystallised plate to be examined 

 may be placed in any azimuth. C is a small moveable 

 stand, by means of which the film to be examined may 

 be placed in any azimuth and at any inclination ; for the 

 usual experiments this is removed. 



" If a lamina of cjuartz cut parallel to the axis, and 

 sufficiently thin to show the colours of polarised light, be 

 placed upon the diaphragm so that its principal section 

 U.c. the section containing the axis) shall be 45° to the 

 lift of the plane of reflexion, on turning the analyser from 

 left to right, instead of the alternation of two comple- 

 mentai-y colours at each quadrant, which appear in the 

 ordinary polarising apparatus, the phenomena of succes- 

 sive polarisation, exactly similar to those exhibited in the 

 ordin.iry apparatus by a plate of quartz cut perpendicu- 

 larly to the axis, will be exhibited ; the colours follow in 

 the order R, O, "\'. G, P., P, V, or, in other words, ascend 



as in the case of a right-handed plate of quartz cut per- 

 pendicularly to the axis. If the lamina be now either in- 

 verted, or turned in its own plane 90°, so that the principal 

 section shall be 45° to the right of the plane of reflexion, 

 the succession of the colour's will be reversed, while the 

 analyser moves in the same direction as before, presenting 

 the same phenomena as a left-handed plate of quartz cut 

 perpendicularly to the axis. Quartz is a positive doubly 

 refracting crystal ; and in it conseijuently the ordinary 

 index of refraction is smaller than the extraordinary index. 

 But if we take lamina of a negative crystal, in which the 

 extraordinary index is the least, as a film of Iceland spar 

 split parallel to one of its natural cleavages, the pheno- 

 mena are the reverse of those exhibited by quartz : when 

 the principal section is on the left of the plane of reflexion 

 the colours descend, and when it is on the rigJit of the 

 same plane the colours ascend, the analyser being turned 

 from left to right. 



" It has been determined that the ordinary ray, both in 

 positive and negative crystals, is polarised in the principal 

 section, * while the extraordinary ray is polarised in the 

 section perpendicular thereto. It is also established that 

 the index of refraction is inversely as the velocity of 

 transmission. It foUoA-s from the above experimental 

 results, therefor-e, that when the resolved ray whose plane 

 of po'arisation is to the left of the plane of reflexion is 

 the quickest, the successive polarisation is right-handed, 

 and when it is the slowest, the successive polarisation is 

 left-handed— in the order R, O, Y, G, B, P, V and in the 

 second case in the reverse order. 



" The rule thus determined is equally applicable to 

 laminK of bi-axal crystals. 



" As selenite (sulphate of lime) is an easily procurable 

 crystal and readily cleavable into thin laminae capable of 

 showing the colours of polarised light, it is most frequently 

 employed in experiments on chromatic polarisation. The 

 lamina; into which this substance most readily splits con- 

 tain in their planes the two optic axes ; po larised light 

 transmitted through such lamina? is resolved in two rec- 

 tangular directions, which respectively bisect the angles 

 formed by the two optic axes ; the line which bisects the 

 smallest angle is called the intermediate section ; and the 

 line perpendicular thereto which bisects the supplemen- 

 tary angle is called the supplementary section. These 

 definitions being premised, if a film of selenite is placed 

 on the diaphragm with its intermediate section to the left 

 of the plane of reflexion, the successive polarisation is 

 direct or right-handed ; if, on the contrary, it is placed 

 to the right of that plane, the successive polarisation is 

 left-handed. The ray polarised in the intermediate sec- 

 tion is therefore the most retarded ; and as that section is 

 considered to be equivalent to a single optic axis, the 

 crystal is positive. 



" In one kind of mica the optic axes are in a plane per- 

 pendicular to the laminx. They are inclined 22.^° on 

 each side the perpendicular within the crystal, but, owing 

 to the refraction, are seen respectively at an angle of 35°'3 

 therefrom. The principal section is that which contains 

 the two optic axes. If the film is placed on the diaphragm 

 with its principal section inclined 45° to the left of the 

 plane of reflexion, the successive polarisation is right- 

 handed. The ray, therefore, polarised in the section 

 which contains the optic a.\es is the one transmitted with 

 the greatest velocity. 



" Films of uni-axal crystals, whether positive or nega- 

 tive, and of bi-axal crystals, all agree therefore in this 

 respect : — that if the plane of polarisation of the quickest 

 ray is to the left of rhe plane of reflexion, the successive 

 polarisation is right-handed when the analyser moves 

 from left to right ; and if it is to the right of the plane of 

 reflexion, other circumstances remaining the same, the 

 successive polarisation is left-handed. 



^ The pfane of polarisatio 

 pciicIicLilar to tli.it of vibrat'n 



tfirougfiout tfiesc pages, taken to be per 



