POLARISED LIGHT. 151 



7i, green light, the result of the coincidences of the 

 waves for yellow and blue light respectively, and the 

 neutralisation by interference of those for red light. 



By substituting Ni col's prisms for the two plates of 

 tourmaline, and by the addition of the object-glass and 

 eye-piece, the diagrams would then represent the passage 

 of polarised light through a microscope. 



For showing objects by polarised light under the micro- 

 scope that are not in themselves doubly refractive, put 

 upon the stage a film of selenite, which exhibits, under 

 ordinary circumstances, the red ray in one position of the 

 polarising prism, and the green ray in another, using a 

 double-image prism over the eye-piece ; each arc will 

 assume one of these complementary colours, whilst the 

 centre of the field will remain colourless. Into this field 

 introduce any microscopic object which in the usual 

 arrangement of the polariscope undergoes no change in 

 colour, when it will immediately display the most brilliant 

 effects. Sections of wood, feathers, algse, and scales, are 

 among the objects best suited for this kind of exhibition. 

 The power suited for the purpose is a two-inch object- 

 glass, the intensity of colour, as well as the separating 

 power of the prism, being impaired under much higher 

 amplification; although in some few instances, such as in 

 viewing animalcules, the one-inch object-glass is perhaps 

 to be preferred. 



Selenite is the native crystallised hydrated sulphate of 

 lime. A beautiful fibrous variety called satin gypsum is 

 found in Derbyshire. It is found also at Shotover Hill, 

 near Oxford, where the labourers call it quarry-glass. Very 

 large crystals of it are found at Montmartre, near Paris. 

 The form of the crystal most frequently met with is that 

 of an oblique rectangular prism, with ten rhomboidal 

 faces, two of which are much larger than the rest. It is 

 usually slit into thin laminae parallel to these large 

 lateral faces ; the film having a thickness of from one- 

 twentieth to the one-sixtieth of an inch. In the two rec- 

 tangular directions they allow perpendicular rays of pola- 

 rised light to traverse them unchanged ; these directions 

 are called the neutral axes. In two other directions, 

 however, which form respectively angles of 45 with the 



