Polarized Light through Boiible-refrading Crystals. 139 



The above are the appearances which present themselves in the 

 case of most films of selenite of a medium thickness. In some cases 

 however, two, or even three, black bands occur simultaneously, 

 these being always followed by as many complementary bands, 

 when the analyzer is turned through 90°. The number of bands 

 can generally be multipHed by using two or more films in com- 

 bination, and the appearances can be still further varied by chang- 

 ing the degree of inclination of the axes of the two films to one 

 another. If the two films are placed with their similar axes coinci- 

 dent, we obtain of course the spectrum appropriate to a film equal 

 to the sum of the thicknesses of the two films, while, if dissimilar 

 axes are superposed, the spectrum is that due to the difierence of 

 the same. I have two films whicli, when properly combined, give 

 no less than six well-marked bands simultaneously. 



But the most striking of the phenomena presented by films 

 which give more than a single band, remains still to be noticed, viz. 

 the motion of the bands along the length of the spectrum. This 

 can generally be easily seen by using two films in combination, and 

 properly adjusting their axes. 



The following may be taken as an illustration of this experi- 

 ment, of which many varieties may be made. (Fig. 1, Plate XOV.) 

 Suppose the two films are so ad.justed as to give two black bands, 

 one in the orange and one in the blue, which we may call a and h 

 respectively. On rotating the analyzer, each band is seen to divide 

 into two halves. The right-hand half of band a runs along the 

 spectrum, and unites with the left-hand half of band h, which 

 advances to meet it, the two coalescing into a single band in the 

 green. At the same time that this has been going on, two entirely 

 new bands have made their appearance. These seem to originate 

 respectively beyond the visible rays at each end of the spectrum, 

 and to advance in opposite directions till they are met respectively 

 by the left-hand half of the original band a and the right-hand half 

 of the original band h. The result is, that when the analyzer has 

 been turned through 90°, we have a spectrum with three black 

 bands, one in the extreme red, one in the green, and one in the 

 indigo. 



Continuing still further to turn the analyzer the above phenomena 

 are reversed. Each of the thi-ee bands split into two, moving in 

 the reverse of their former directions, until when 180° is reached 

 the original spectrum with its two bands recurs. 



A curious variety of this experiment occurs when a circularly 

 polarizing film is interposed between the analyzer and the film 

 producing the bands. The nature of the movements of the bands 

 is now entirely changed, the order of motion being all in the same 

 direction, and the bands appearing to chase one another along the 

 length of the spectrum, making their appearance at one end and 



