110 THE MICROSCOPE. 



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 microscope, put 

 upon the stage of a film of selenite, which exhibits, under ordinary cir- 

 cumstances, the red ray in one position of the polarising prism, and the 

 green ray in another ; each arc will assume one of these complemen- 

 tary colours, whilst the centre of the field will remain colourless. Into 

 this field introduce any microscopic object which in the usual arrange- 

 ment of the polariscope undergo no change in colour, when it will 

 immediately display the most brilliant effects. Sections of wood, fea- 

 thers, 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 in- 

 tensity of colour, as well as the separating power of the prism, being 

 impaired under much higher amplification ; although in some few in- 

 stances, such as in viewing animalcules, the inch-object-glass is perhaps 

 to be preferred. 



Mr. J. King attributes the foregoing phenomenon to the double- 

 image prism separating the constituents of the polarised ray into two 

 planes, and causing them to overlap each other, except at the edges 

 where the light is analysed ; and of course, the combination of comple- 

 mentary colours in the centre of the field produces white light. Any 

 object, therefore, placed in the white field, partakes of the characters of 

 the selenite ; one image being refracted into the plane which exhibits 

 the green ray, assumes that colour ; whilst the second image, being re- 

 fracted into the plane of the red ray, partakes of that tint. Should the 

 object, however, be so large that actual separation of the images is not 

 effected, the extremities and interstices only will be polarised, whilst 

 the middle will remain dark, or present only its natural hue. 



Selenite is the native crystallised hydrated sulphate of lime. A beau- 

 tiful fibrous variety called satin gypsum is found in Derbyshire. It is 

 found also at Shotover Hill, near Oxford, where the labourers call it 

 qua/rry-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 rectangular direc- 

 tions they allow perpendicular rays of polarised light to traverse them 

 unchanged ; these dir^tions are called the neutral axis. In two other 



