Number and Nature of the Optical Axes of Glauber ite. ^19 



point, the order and connexion of the phaenomena will be 

 more easily understood. 



At 32° glauberite has two axes of double refraction for 

 rays of all colours, the inclination of the axes for the violet 

 rays being least, and that for the red the greatest. As the 

 temperature rises, the optic axes for all colours gradually ap- 

 proach, and the axes ibr violet first unite into one. At this 

 time the crystal has two axes for all the other colours; but 

 as the heat increases, all the other pairs of axes unite in suc- 

 cession, and form a single system of rings. But befoi'e this 

 has taken place, the axes for violet rays have opened up again 

 in a plane at right angles to that in which they originally hiy, 

 •and they are followed by all the other pairs of axes ; so that 

 at a temperature much below that of boiUng water, each pair 

 of axes appears with different inchnations arranged in a new 

 direction. 



During all the clianges which have been described above, 

 the crystal has preserved its constitution, and by abstracting 

 the heat, the phfenomena are all repeated in an inverse order. 



If the crystal should happen to be observed at that tempe- 

 rature, which very often occurs, when the greenish -yellow or 

 most luminous rays have the optic axes corresponding to them 

 united, or form a single system of rings, then the blue rays 

 will have two systems of rings lying in one plane, and the red 

 rays also two systems of rings in a plane at right angles to 

 this. In two rectangular positions, namely, when the planes 

 of the double axes coincide with, or are at right angles to, the 

 plane of primitive polarization, the black cross will be very 

 distinct, but in intermediate positions it will be much less so, 

 and the uniaxal system of rings which predominates, from the 

 greater intensity of their light, will have that indistinctness of 

 character which, whenever it occurs, indicates a peculiar action 

 of the doubly refracting force on the differently-coloured rays. 

 When the black cross is perfect and equally distinct in all 

 positions, while the colours of the rings deviate from those of 

 Newton's scale, then the axes for all colours are obviously 

 coincident, and the peculiarity in the colour of the rings is 

 owing to an irrationality hi the action of the doubly-refracting 

 forces on the differently-coloured rays. This deviation from 

 the tints of Newton's scale, I have found in many crystals 

 which have only one axis of double refraction. It is extremely 

 common in crystals with two axes. 



I have elsewhere described the construction of a chromatic 



thermometer, in which the temperature is indicated by the 



polarized tints transiently developed by heat in a number of 



plates of glass; — but it is obvious that a plate of glauberite 



3 H 2 



