122 POLAPJSCOPE. [CH. V 



existing in the substance of the cell-walls or the starch 

 grains as a result of their particular structure. In order 

 to realise that tension may produce double refraction in a 

 substance that is not in itself anisotropic, for example 

 glass, a fine glass filament should be forcibly extended, 

 while it is under observation in polarised light in the field 

 of the microscope. To make the effect more apparent use 

 should be made of the selenite discs generally supplied 

 with the polarising apparatus. The various colours which 

 bodies exhibit when viewed in these coloured fields give a 

 measure of the strength of their double refraction. To 

 perform the stretching experiment a piece of glass rod, 

 drawn out at the blow-pipe to a fine filament in its middle 

 part, should be so clamped at one end that the fine part 

 lies across the field of the microscope and can be focussed 

 with a low power. The selenite disc No. I., which gives a 

 red-purple field, should be suitably placed below the glass 

 thread, which then appears as a double black contour 

 with red-purple between. The free end of the glass rod 

 should rest on some guiding support which will keep it in 

 focus but allow it to be stretched. If the observer 

 looks down the microscope while the rod is steadily 

 pulled, the colour of the centre of the thread will 

 be seen to change distinctly, the nature of the change 

 depending on the amount of traction exerted upon 

 the filament: on releasing it the purple colour re- 

 appears. 



Compression should give a different colour-change but 

 this cannot be easily exhibited. Other transparent bodies 

 such as gelatine films show similar effects. 



