POLARISED LIGHT 359 



a small crystal held in the end of a tube ; if the conditions 

 are hit rightly, which can only be done after practice, the 

 effect is very fine. In the polarising projection microscope, 

 the best plan is to purchase two or three slides of the fatty 

 acids prepared for this purpose ; they have simply to be gently 

 warmed till the crystals all melt away, and placed on the 

 stage, when they crystallise out again as the slide cools. The 

 same slide can be used dozens of times over with no further 

 treatment. 



207. Mineral Sections. Every one of these can be well 

 exhibited in the polarising projection microscope, but only 

 the coarse-grained specimens are bold enough for the polari- 

 scope. Among those I know to be effective, are the coarse 

 sandstones, granites, perthites, zeolites, cross sections of 

 small stalactites, and labradorite. Some minerals show 

 interesting and extraordinary differences in figure as well as 

 colour, when cut in different planes. Labradorite cut in one 

 direction, shows little but a coloured film ; in another, 

 beautiful straight coloured stripes ; and in a third, beautiful 

 bands of rotational colours ( 213). One kind of granite 

 (often called * graphic ' granite) resembles ordinary granite 

 cut in one plane, whilst in another it displays marking almost 

 like an Arabic inscription. 



208. Organic Substances. Nearly all substances with 

 definite structure show double refraction when cut into plates 

 or films. For the lantern polariscope, the best objects are 

 plates of thin horn, pieces of thick bladder, and quill. If 

 quills are split up one side, and placed in boiling water, they 

 become soft and can be flattened out ; but a quill pen just as 

 it is, pushed through the stage, will show the phenomena 

 plainly. Where there is fibre in a definite direction, as in a 

 quill, one polarising plane always lies that way, and the other 

 at right angles. Therefore if the quill be placed parallel 

 to or across the polariser, the field remains dark, and the best 

 effect is produced at an angle of 45, as with crystalline films. 



