140 TRANSACTIONS OF THE 



carbonate of lime, which crystallizes in rhombs and gives a double 

 reflection. Mr. Nicol first succeeded in making single image 

 prisms of Iceland spar, which reflect only the ordinary or the 

 extraordinary ray at the time. A polariscope consists of two 

 such prisms, one of which is placed under the stage below the 

 object and is called the Polarizer, and the otlier is either placed 

 behind the object-glass or above the eye-piece and is called the 

 Analyzer ; both Polarizer and Analyzer are made to turn. If then 

 an object such as doubly refractive ciystals are placed on the 

 stage, and either of the prisms is turned, they will exhibit the 

 polarization of light in gorgeous colours. If objects are only 

 simply refractive they require the interposition of a film of 

 selenite, which according to its thickness will exhibit two com- 

 plementary coloui-s such as red and green, blue and yellow, &c. 

 By combining several selenites a large variety of colours and tints 

 may be obtained during half a revolution. Selenite is the native 

 crystallized hydrated sulphate of lime. The best arrangement of 

 selenites is Barker's, who places three selenite films of various 

 thicknesses in a box in which they can be rotated both singly and 

 in combination with each other. The variety of tints of a selenite 

 may be greatly increased by interpolating a rotating film of mica, 

 and if one such mica film is combined with two selenites, the one 

 blue and yellow and the other red and green, the entire series of 

 tints which are produced by any niimber of selenites combined can 

 be obtained. The polariscope may be combined with the spot-lens 

 or paraboloid for a dark-ground polariscope. If the analyzer consists 

 of a double image prism or an original rhomb of Iceland spar, and if 

 a brass plate with a small hole be placed on the stage, two magni- 

 fied images of the latter will be seen which exhibit complementary 

 colours, and if the hole be large enough the two images will partly 

 overlap each other, and that portion where they overlap will look 

 white, thus proving that the complementary colours produce white 

 light. The objects especially suited for the polariscope are 

 chemical crystals, hairs, fish scales, sections of horns and hoof, starch 

 grains, vegetable and muscular fibres, the palates or lingual bands 

 of molluscs, &c. Crystals are thus seen as they cannot be seen 

 otherwise, and the chemically distinct constituents of vegetable 

 and animal tissues become apparent in a way which no mere 

 magnifying power could resolve. By the addition of a polariscope 

 a great increase of power is thus gained. 



