288 SENARMONT ON THE OPTICAL CFIARACTERS OF 



the violet outwards. There is one position in which the red axes 

 again unite. The crystal has then only one axis for the red rays, 

 and for other colours two axes, which diverge more and more up 

 to violet; these latter are moreover at least 12° apart in original 

 planes. 



From this moment the red optical axes separate in their turn, 

 but in the diagonal plane perpendicular to the first, while the 

 violet axes continue to approach. The complex crystal then 

 contains ammonia salt by the position of the red optical axes, 

 potash salt by the position of the violet optical axes, and has 

 only one axis for the intermediate colours. Lastly, inasmuch as 

 the optical axes continue to suffer the same alteration of position 

 in proportion as the influence of the ammoniacal salt increases, 

 the violet axes pass after the red axes into the plane normal to 

 that in which they are at first situated, and ultimately form in 

 this plane an angle of 46° or 48°, while the red axes form an 

 angle of 60°. The optical character of the potash salt gradually 

 disappears, and is superseded by that of the ammoniacal salt. 



The gradual transformations which the dispersion of the op- 

 tical axes for different colours suffers in the mixed salts do 

 not take place without giving rise to very curious distributions 

 of colours in the isochromatic curves, and such contortions of 

 them that it is impossible to recognise their ordinary outlines. 

 The apparent confusion nearly reaches its maximum when the 

 red and violet axes situated in tw^o rectangular planes respectively 

 form with each other an angle of from 6° to 7°- However, the 

 ordinary uniformity is restored in the appearance of the rings 

 when homogeneous light is employed, such for example as that 

 which is transmitted through red or dark green glass, or a con- 

 centrated solution of carbonate of copper and ammonia. 



The contortion of the isochromatic curves is moreover accom- 

 panied by certain remarkable alterations of colour in the central 

 region of the rings, when the analyser is rotated. 



For example, when the analyser is turned from left to right, 

 the colours successively seen in the field are red, yellow, green, 

 blue, and violet, following each other almost in the same manner 

 as the colours of the spectrum. These phaenomena present a 

 considerable resemblance to those which result from circular 

 polarization ; it is nevertheless obvious that they are, at least to 



