POLARISED LIGHT THROUGH DOUBLE REFRACTING CRYSTALS. 179 



To understand the bearing of the experiments, it is necessary to keep in 

 view the different effects of a doubly refracting film upon polarised light, accord- 

 ing to the position of its axes, with respect to the planes of polarisation. 



Suppose we take a film of selenite, such as those commonly sold as an 

 adjunct to the polarising microscope, giving, as its two colours, a pinkish red 

 and its complementary green. Such a film will, if examined between two 

 Nicol's prisms, act on the light according to the following laws : — 



1st. When a neutral axis of the film is in the plane of primitive polarisation, 

 the film will exercise no influence on the light ; if, therefore, the prisms are set 

 with their axes perpendicular the field will remain dark, if the prisms have 

 their axes parallel the field will contain only white light. 



2d. If the prisms are placed with their axes perpendicular, and the film 

 is made to rotate, there will be four points of darkness at each quarter of a 

 revolution, viz., when an axis of the film is in the plane of polarisation, and 

 between these four points, the same colour (say green) will occur. 



3d. If the prisms are set with their axes parallel, and the selenite is rotated, 

 the field will be white at the four points where it was previously dark, and of 

 the complementary colour (red) between each of these four points. 



Mh. If the selenite is fixed with its neutral axis inclined 45° to the plane of 

 primitive polarisation, and the analyser made to rotate, the field will be alter- 

 nately red and green in the four quadrants. 



5th. The colours are always of maximum brightness when the axes of the 

 prisms are perpendicular or parallel, and the axes of the selenite inclined 45° to 

 the plane of polarisation. 



Suppose, now, we repeat the above experiments, using the polarising spectrum 

 microscope above described, and let us call the point in the revolution of the 

 selenite at which either of its axes is in the plane of primitive polarisation, the 

 zero point, from which the number of degrees through which it is turned are 

 measured. 



Let the prisms be set with their axes perpendicular to one another, and the 

 selenite rotated on the stage. The spectrum will, of course, vanish at the four 

 zero points. Between these points, however, remarkable phenomena occur. A 

 person unacquainted with the true nature of the colours of polarisation, and 

 proceeding on the analogy of homogeneous light, might expect to get a spectrum 

 consisting only of green rays, seeing that that is the colour of the field when the 

 spectrum arrangement is removed. This, however, is not the case, and the 

 result very beautifully illustrates to the eye what is well known theoretically to 

 be the true nature of these colours. What we obtain is a continuous spectrum 

 consisting of all the prismatic colours, in greater or less intensity, with the 

 striking peculiarity that there is a well-marked dark band in the red, similar in 

 appearance to the well known absorption bands which many substances pro- 



