16 



POLARISATION OF LIGHT. 



with wax, so that no dust may introduce 

 itself between the plates. If the glass 

 is thin and with little colour, the light re- 

 flected from its surface will be as bright 

 as that reflected from a quicksilvered 

 mirror, and will consist wholly of polar- 

 ised light, when the rays are incident 

 upon it, at the polarising angle. The 

 light transmitted may be also used. 



3. Reflecting Bundles of Blown Glass. 

 As it is extremely difficult to obtain 

 thin plates of clear and colourless glass, 

 even if we take flint glass, which is not 

 desirable from its high dispersive power, 

 we may substitute in their place films of 

 glass blown to the utmost thinness, and 

 place them in a trough between two 

 plates of the thinnest glass. The light 

 transmitted through this bundle may 

 be also used. 



4. Reflecting Bundles of Mica. Take 

 a piece of clear and transparent mica, 

 as colourless as possible, and cut it into 

 the form of a right angled parallelogram, 

 whose sides are parallel and perpendi- 

 cular to the plane passing through its 

 resultant axes*. Hold it by an edge in a 

 powerful vice, and with a lancet, or a 

 thin-bladed knife, split it into ten or 

 twelve laminae, or more if necessary. 

 Before taking it out of the vice, cover all 

 its edges with a coating of wax or strong 

 cement, so that, after the laminae are 

 separated from one another, they may 

 have the same relative position as before 

 their separation. This bundle of mica 

 films, when taken out of the vice, is 

 one of the best means of polarising light 

 that can be used ; but the light must be 

 polarised by reflexion in a plane parallel 

 to either of the sides of the bundle. 

 The best is that which is perpendicular 

 to the plane passing through the re- 

 sultant axes. The light transmitted 

 through this bundle being also perfectly 

 polarised, may be used with great ad- 

 vantage. 



5. Doubly -refracting Crystals of great 

 Thickness. When we can obtain a 

 thickness of from three to six or more 

 inches of colourless calcareous spar, it 

 forms one of the most valuable pieces 

 of polarising apparatus. We have only 

 to place on one of the sides that contains 

 the greatest thickness, a circular aperture 

 just as large as that the two images of 

 it may not overlap each other. We 

 shall thus have two circular areas of light 

 which are polarised, the one in one plane, 

 and the other in a plane at right angles 

 to it; and by means of a screen or a 



next chapter ' lU ^ Understood after Pausing the 



black wafer we can cover up the one 

 circular space, when we require only one 

 kind of polarised light. 



6 . Doubly-refracting Prisms of Iceland 

 Spar. As it is not easy to procure large 

 and pure masses of Iceland spar, a suffi- 

 cient separation of the images may be ob- 

 tained, by selecting a piece with one good 

 natural surface, and grinding down the 

 other, so that the common intersection of 

 the two faces of the prism may be per- 

 pendicular to the axis or the plane of 

 refraction, coincident with the plane of 

 its principal section. The colour of the 

 images may be nearly corrected by a 

 prism of crown or flint glass*. By in- 

 creasing the refracting angle of the 

 prism, the separation of the images may 

 be increased f at pleasure. The objec- 

 tion so often made to the use of prisms 

 of Iceland spar is not well founded ; for 

 it is capable of taking an admirable 

 polish, equal indeed to its original sur- 

 face ; and even if the operator is not 

 skilful in the art, the polish may be 

 made perfect, and the surface preserved 

 from injury, by cementing, on the two 

 surfaces of the prism, two pieces of pure 

 and parallel glass. When the prism 

 is rendered achromatic, indeed, by a glass 

 prism, the latter serves for one of the 

 plates of glass, and one plate only is re- 

 quired for the other surface. We have 

 had prisms of this kind which have 

 lasted fifteen years, though exposed to 

 constant use. The separation of the 

 images will be a maximum with the 

 same refracting J angle if the faces of 

 the prism, or their common intersections, 

 are parallel to the axis of double re- 

 fraction. 



7. Doubly -refracting Prisms of Rock 

 Crystal. The following ingenious me 



Fig. A. 



* In the Annals of Philosophy for March, 1818, 

 p. 175, Dr. Brevvster has described a method of com- 

 pletely correcting the two dispersions of the two 

 images of Iceland spar, by using two prisms of crown 

 glass of different refracting angles, and making the 

 rays which form one image pass through one prism, 

 and the other rays through the other prism. 



