182 FRANCIS DEAS ON THE SPECTRA FORMED BY THE PASSAGE OF 



prism. Two spectra formed respectively by the ordinary and extraordinary 

 ray are thus obtained, which by rotating the double image prism may be made 

 to lie parallel to one another, or be partially superposed at pleasure, while by 

 turning the polarising prism the spectra can be made of any desired relative 

 intensity. Suppose that we adjust the two prisms with their axes at right 

 angles, and interpose the selenite used in the first experiment, which gave a 

 band alternately in the red and in the green, we get now two spectra parallel 

 to one another, the band in the red of the one occurring simultaneously with 

 the band in the green of the other. The two bands are thus seen to be strictly 

 complementary, for the band in the red of the one spectrum appears, attains its 

 maximum, and vanishes simultaneously with the similar changes of the band in 

 the green of the other spectrum. This coincident appearance of the bands, 

 moreover, is independent of the inclination of the axis of the selenite to the 

 plane of polarisation, the only effect of a change in which is to increase or 

 diminish the maximum intensity of both bands alike, a result which, as has 

 been noticed, does not hold with regard to the alternation of the two bands in 

 the same spectrum.* 



When the two prisms are placed with their axes parallel, so that the two 

 images of the slit are seen alongside one another, and consequently the two 

 spectra partially superposed and different colours mixed, the appearance of the 

 bands is very striking. A band occurring in either spectrum is no longer black, 

 but of the colour of that part of the other spectrum which coincides with it. 

 The appearance is, in fact, as if a stripe had been cut out of the one spectrum 

 through which the colour of the other spectrum is seen, while on either side of 

 the band we have in striking contrast the colours due to the compounding of the 

 different parts of the two sjDectra. 



The beauty of the effect depends of course greatly on the extent to which 

 the double image prism separates the two images. It should be so cut that the 

 compound colours caused by the overlapping of the spectra shall be as different 

 as possible from either of their constituent colours. The selenite should then 

 be set at 45°, so as to make the spectra of equal and the bands of maximum 

 intensity. 



With films which give numerous bands the effect is very beautiful, and may 

 be still further enhanced by rotating the polariser, when the bands will shift 

 their position, at the same time changing their colours. 



Experiments with Sections of Double Refracting Crystals giving Coloured Rings. 



The coloured rings produced when polarised light is transmitted through a 

 double refracting crystal cut perpendicularly to its axis, have always been 



* See Note on p. 180. 



