POLARISED LIGHT THROUGH DOUBLE REFRACTING CRYSTALS. 181 



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 experiment, of which 

 many varieties may be made. Suppose the two films are so adjusted as to give 

 two black bands, one in the orange and one in the blue, which we may call a 

 and b respectively. On rotating the analyser, 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 b, 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 b. The result is, 

 that when the analyser 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 analyser the above phenomena are re- 

 versed. Each of the three bands splits 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 polarising film 

 is interposed between the analyser 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 disappearing at the other. To produce this effect, the " band-producing" 

 film should be set with its neutral axis at 45°, and the circularly polarising film 

 superposed with its neutral axis in the plane of primitive polarisation. If the 

 axis of either film is turned through 90°, the motion of the bands is reversed ; 

 i.e., if the bands formerly moved from left to right, they now move from right 

 to left. If the two films are both placed with their axes at 45° to the plane of 

 polarisation, the only effect of the circularly polarising film is to alter the posi- 

 tion of all the bands by a corresponding amount [i.e., to increase or diminish 

 their refrangibility) without affecting the nature of their motions.* 



A very pleasing and beautiful variety of the foregoing experiments may be 

 obtained by using a double image prism as the analyser instead of the Nicol's 



* The effect of circularly polarising the light before it passes through the selenite, is simply that 

 the occurrence of the hands is irrespective of the inclination of the axis of the selenite to the plane 

 of primitive polarisation, and depends solely on the position of the analyser. 



VOL. XXVI. PART I. 3 B 



