110 POLARISCOPE. [CH. V 



(141) Tension. 



All crystalline bodies are anisotropic and it is one view 

 of the significance of the anisotropism of organised bodies, 

 cell- walls, starch grains, etc., to attribute this to a 

 crystalline structure of the ultimate particles — micellae of 

 Nageli — of which these bodies are held to be built up. 



Another view denies this crystalline structure and 

 attributes the anisotropism to the tensions produced 

 between the strata of the substance of the cell-wall or the 

 starch grain 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, the experiment should be performed of stretching a 

 fine glass filament 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. On 

 placing one of these on the stage of the microscope the 

 field will appear of a certain colour which changes on 

 rotation of the analyser. The various colours which 

 bodies exhibit when viewed in these coloured fields give a 

 measure of their respective anisotropism, but the theory 

 of this cannot be entered into here. To perform the 

 stretching experiment a piece of glass rod, drawn out at 

 the blowpipe 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, 



