POLARISED LIGHT AND THE MICROSCOPE. 267 



diately ; but if not, if you get out feeling cold and shivery, do not 

 continue it, but add a little warm water to make it tepid, or merely 

 sponge with tepid water, and rub yourself well with a dry towel. 

 Its value consists in exercising the cutaneous blood-vessels to con- 

 tract and relax rapidly, when exposed to sudden changes in the 

 temperature. 



Lastly, please remember that feeling a sensation of warmth 

 does not mean that the body temperature is raised, but, in itself, 

 shows that, by the glowing skin, the necessary process of cooling 

 is taking place. 



Ipolaiiecb XiQbt an& (te applicatione to 



the fllMcroecope. 



By G. H. Bryan, M.A. 

 Part III. 



Chromatic Polarisation.— We will now examine the cause of 

 the pretty colours displayed by certain polariscopic objects, and 

 more especially by crystals, films of selenite, and rock sections. 

 To exhibit good colours, the object must be neither too thi7i nor too 

 thick. When a rock section has been ground fairly thin, it shows 

 splendid colours ; but on grinding it still thinner and thinner, all 

 the colours disappear, and at last the only effect remaining is 

 that certain portions of the section appear greyish white when the 

 polariscope is adjusted to give a dark background. 



In preparing crystals for the polariscope, too, they must not be 

 allowed to form either too thick or too thin a coating on the slide 

 if pretty colours are desired. If too thin, the crystals will appear 

 of a greyish white colour on the dark background ; if very thick 

 indeed, they will again appear white; but the best colours will be 

 obtained with certain intermediate thicknesses. 



Again, the thinner animal hairs only appear white when the 

 polariser and analyser are crossed ; but thicker hairs, such as 

 those of the Polar Bear (which is thus " polar " in two senses of 

 the word), exhibit fine colours. 



The Colours of the Spectrum.— To explain these colours by 



