142 On Spectra formed hy the passage of 



maximum intensity, and the spectrum with its series of rings is 

 seen to be cut in two by tlie jet-black brushes. WTien the analyzer 

 is turned through 90^ the brushes which would now, if seen by 

 ordinary polarized light, be white, are of every colour in the spec- 

 trum according to the part of it they fall upon, and shaded off at 

 their sides by a nebulous haze of colour through which the black 

 rings are visible. 



In intermediate positions of the analyzer the brushes become 

 entirely nebulous, so that the rings can be seen through their 

 whole extent. In this position of matters the circle appears divided 

 into four quadrants, and the rings are distinctly seen to be dis- 

 located so to speak, i. e. the rings in the alternate quadrants are 

 pushed out so that each coloured rmg in the one quadrant is 

 continuous with a black ring in the next. 



This effect is still better seen by circularly polarizing the light 

 before its passage through the crystal. (Fig. 4, Plate XCV.) The 

 effect of this is a curious one. Instead of the circle being divided 

 into four alternate quadrants, it is now divided into two semicircles, 

 the rings in the one being alternate with those in the other. The 

 semichcles are separated by two narrow coloured brushes which 

 revolve with the analyzer, and seem as if they swept* out the black 

 rings in the one segment to be replaced by the coloured rings of 

 the next. If we again circularly polarize the light by interposing a 

 second circularly polarizing plate between the crystal and the 

 analyzer, the brushes entirely disappear, and both the black and 

 the coloured rings are continuous throughout, forming perfect 

 circles. 



When the analyzer is rotated through QO'^, the centre of the 

 system which was formerly black is now coloured, and, at the same 

 time, all tlie black rings have exchanged places with the coloured 

 rings, the change being effected by a lateral displacement in opposite 

 dhections of the two halves of the circle. 



If for the second chcularly polarizing film we substitute a film 

 of a different thickness, the rings assume curiously distorted forms. 

 With one film which I used the rings became ellijjses, with another 

 they all united so as to form a circular hehx, which appeared to 

 unwind like a screw as the analyzer was turned. 



The appearances produced by using different crystals are of 

 course similar, mutatis mutandis. 



By circularly polarizing the light before and after its passage 

 through a crystal of nitre (Fig. 5, Plate XCV.), the brushes are 

 wiped out, and the lemniscates are beautifully seen, unbroken 

 throughout. 



When a crystal of Iceland spar is used, and the Nicol's prisms 

 set with their axes inclined 45° we get eight segments, of which 

 the four light segments look as if they stood out in relief against 



