POLARISED LIGHT 379 



medium which will preserve these crystals clear is much to 

 be desired. When clear, by placing in the stage gelatines 

 carefully selected to absorb all but the blue and red rays, 

 the two systems of rings can be shown crossed on the screen 

 if the light is brilliant. The axes are shown easily. 



Airy's Spirals are produced by a right-handed quartz, cut 

 perpendicularly to the optic axis, superposed upon a left-handed 

 one. The two need not be of equal thickness, and plates 

 containing cross-crystallisation will show the spirals naturally, 

 wherever the two forms happen to overlap each other. 



It ought to be noted, that crystals which can be shown 

 in both forms of polariscope, require to be much thinner for 

 highly convergent light than for the low convergence method. 

 A thick plate of nitre will fill the field, with low convergence ; 

 for high convergence it must be very thin, and the two axes 

 will appear very close together in the centre of the field. 

 Most mica commonly obtainable has an angle of about 45. 



Mitscherlich's Experiment, showing that a crystal of 

 selenite, when heated, gradually becomes uni- axial, and with 

 further heat becomes again bi-axial with the axes at right 

 angles to the former direction, is easily demonstrated with 

 the apparatus described. I employ a slide devised for the 

 purpose ; but it is sufficient to procure a strip of brass or copper 

 about 4x1 inches, the same thickness as the crystal, drill 

 and file a hole through the centre which just admits the latter 

 loosely, and bend up rather more than an inch of each end, 

 so that these ends stand well away from the crystal stage like 

 ears, when the strip is held by the springs. Then bend a piece 

 of thin card round one edge, so as to cover both sides of the 

 centre of the strip, and cut through both cards a hole rather 

 smaller than the crystal ; thus the card, as it embraces the 

 brass with the crystal in it, keeps the latter in place. All this 

 being adjusted on the stage, and the rings focussed as usual, 

 a lighted spirit-lamp is taken in hand and applied alternately 

 to the projecting ears. At first a slight mist generally appears, 



