36 



POLARISATION OF LIGHT. 



would yet be found in which the axis is 

 negative in all the rays of the spectrum* ; 

 and some years afterwards he discovered 

 the remarkable mineral of oxahverite, 

 which is an apophyllite with this property .t 



These views have been confirmed and 

 illustrated by a more recent observation 

 of the same author, who has found that 

 glauberite has two axes for red light and 

 only one negative axis for violet light. 

 In this case the single negative violet 

 axis C is the resultant of two positive 

 axes at A and B of equal intensity, while 

 the same two axes have different inten- 

 sities for red and the other rays of the 

 spectrum. 



Hence in apophyllite the single system 

 of rings is the resultant system of three 

 rectangular axes, while in glauberite 

 the single system of violet rings is the 

 resultant of two rectangular axes. 



CHAPTER X. 



Cause of the Colours of Polarised 

 Light Blot's Theory of moveable 

 Polarisation Laws of the Inter- 

 ference of Polarised Rays their ap- 

 plication to explain the Colours of 

 Polarised Light. 



HAVING thus described, as briefly and 

 perspicuously as we can, the general 

 phenomena of the colours of polarised 

 light as produced by doubly refracting 

 crystals, we shall proceed 'to consider 

 the explanations which have been given 

 of them. We have already shown, in 

 Chap. VI. p. 19, that the thin plate of 

 sulphate of lime, D E F G, divides 

 white light into two coloured pencils, 

 complementary to each other, as, for ex- 

 ample, red and green. These are the ex- 

 traordinary and ordinary images pro- 



Fig. 39. 



duced by double refraction ; and we shall 

 distinguish them by the letters E and O. 

 In doubly refracting plates or crystals 

 of considerable thickness, the two pen- 

 cils, O and E, are perfectly white and 

 equal, and are polarised in planes at 

 right angles to each other, as already 

 explained; but in thin plates, where O 

 and E are coloured, they are polarised in 

 a different manner. 



Since the extraordinary pencil, namely, 

 the red one or E, is reflected by the ana- 

 lysing plate C, and is a maximum when 

 C is in the azimuths of 90 (the one shown 

 in the figure) and 270, its polarisa- 

 tion must be different from that of the 

 ray A B ; and since no part of the ordinary 

 pencil, or ihegreen one O, is reflected, this 

 last must have the same polarisation as 



* Edin. Kncycl. art. Optics. Vol. XV. p. 5^7. 

 t Edin. Journal of Science. No. Xlil. p. 115. 



the pencil A B. Hence the action of 

 the plate of sulphate of lime, D E F G, 

 upon the polarised ray A B, is to divide 

 it into two pencils, O and E, green and 

 red, the former having the same polar- 

 isation as A B, and the" latter a different 

 polarisation. Let us now suppose the 

 plate D E F G to revolve round the ray 

 A B, and let a be the angle of azimuth 

 which the axis F G of the crystal or plate 

 of sulphate of lime forms with the plane of 

 primitive polarisation R A B : then we 

 have already seen that when a = 45, the 

 red rays are reflected at C, when the 

 azimuth of C is 90 and 270, and is not 

 reflected at all when its azimuth is and 

 180. Hence the red pencil or E must 

 have been polarised in a plane at right 

 angles to that of A B, or the change of po- 

 larisation effected by the plate mu^t have 

 been 90 = 2 x 45 = 2 a. By making a of 



