326 



NATURE 



\Feb. 2 



" It is for this purpose much more convenient than a 

 Fresnel's rhomlj, with which but a very small field of view 

 can be obtained. It must, however, be borne in mind 

 that the circular and elliptical undulations are inverted in 

 the two methods : in the former case they undergo only 

 a single, in the latter case a double reflexion. 



" For the experiments which follow, the ciystallised 

 plate must be placed on the diaphragm E, between the 

 silver plate and the analyser, instead of, as in the preced- 

 ing experiments, between the polariser and the silver 

 plate. 



Fig. 15 — Wheitstoiit. b modiht'xtion of Norremberg s Polaribcope. 



" By means of a moving ring within the graduated 

 circle D, the silver plate is caused to turn round the re- 

 flected ray, so that, while the plane of polarisation of the 

 ray remains always in the plane of reflexion of the glass 

 plate, it may assume every azithumal position with respect 

 to the plane of reflexion of the silver plate. The film to 

 be examined ; nd the analyser move consentaneously 

 with the silver plate, while the polarising mirror remains 

 fixed. 



" In the normal position of the instrument the ray 

 polarised by the mirror is reflected unaltered by the 

 silver plate ; but when the ring is turned to 45°, 135°, 

 225°, or 315", the plane of polarisation of the ray falls 45° 

 on one side of the plane of reflexion of the silver plate, 

 and the ray is resolved into two others, polarised lespec- 

 tively in the plane of reflexion and the perpendicular 



plane, one of which is retarded on the other by a quarter 

 of an undulation, and consequently gives rise to a circular 

 ray, which is right-handed or left-handed according to 

 whether the ring is turned 45" and 225°, or 135° and 315°. 

 When the ring is turned so as to place the plane of polari- 

 sation in any intermediate position between those pro- 

 ducing rectilinear and circular light, elliptical light is 

 obtained, on account of the unequal resolution of the ray 

 into its two rectangular components. 



" Turning the ring of the graduated diaphragm from 

 left to right, when the crystallised film is between the 

 silver plate and the analyser, occasions the same succes- 

 sion of colours for the same angular rotation as rotating 

 the analyser from right to left when the instrument is in 

 its normal position and the film is between the polariser 

 and the silver plate." 



The same principles apply to the case of bi-axal crys- 

 tals cut parallel to a plane containing the two optic axes. 

 A ray of plane-polarised light transmitted through such a 

 plate is divided into two, whose vibrations respectively 

 bisect the angles formed by the two axes. As mentioned 

 above, the line which bisects the smallest angle is called 

 the intermediate section, and the line perpendicular to it 

 the supplementary section ; and the order of the colours 

 depends upon the relative velocity of the two rays. In se- 

 lenite, the ray whose vibrations lie in the supplementary 

 section is the slowest ; in mica it is the swiftest. Hence 

 these two crystals, all other circumstances being alike, give 

 the colours in opposite orders, and may be regarded as 

 positive and negative, like quartz and Iceland spar. And a 

 test similar to that indicated for uni-axal may be apphed 

 to bi-axal crystals. 



Some interesting and varied experiments may be made 

 by using two circularly polarising instruments, e.g., two 

 quarter undulation plates (say the plates A and B), one 

 between the polariser proper and the ciystal (C) under 

 examination, the other between the crystal and the 

 analyser. The light then undergoes the following pro- 

 cesses. If the plate A be placed so that its axis is at 45'' 

 on one side or other of the original plane of vibration, 

 and the plate B with its axis parallel or perpendicular to 

 that of A, then on turning the analyser we shall have the 

 phenomena of circular polarisation described above. 

 Again, if, the plates A and B retaining the positions 

 before indicated, the crystal C be turned round in its own 

 plane ; then, since the light emerging from A and B is 

 circularly polarised, it has lost all trace of direction with 

 reference to the positions of polariser and analyser, and 

 consequently no change will be observed. 



Agam, if the plates A and B have their axes directed 

 at 45" on either side of the axis of C, and the three plates 

 be turned round as one piece, the colour will remain un- 

 changed, while if the analyser be turned, the colours will 

 follow in the regular order. If the plates A and B have 

 their axes directed at 45° on the same side of the axis of 

 C, and the pieces be turned round bodily as before, the 

 colours change in the same order as above, and go 

 through their cycle once in every right angle of rotation ; 

 and if the analyser be turned in the same direction, the 

 colours change, but in the reverse order. The explana- 

 tion of this is to be found in the fact that when the plates 

 A and B are crossed, the retardation due to A is compen- 

 sated by that due to B ; so that the only eft'ective retarda- 

 tion is that due to the crystal C. But upon the latter de- 

 pends the rotation of the plane of vibration ; if, therefore, 

 the polariser and analyser remain fixed, the colour will 

 remain unaltered. When the plates A and B have their 

 axes parallel, there is no compensation, and the colour 

 will consequently change. It should be added that the 

 rotation of the plane of vibration, and consequently the 

 sequence of colours, does not follow exactly the same law 

 in these cases as in quartz. 



W. Spottisvvoodf. 

 {To be continued.) 



