ON POLARIZED LIGHT. 193 



of an inch thick), so that the polarized beam may pass through 

 it perpendicularly. It should be placed between the polarizer 

 and the analyzer, as on the stage of the microscope. If now 

 the eye is applied to the polarizing apparatus, as before, the 

 surface of the film of sulphate of linie, &c., will be seen 

 covered with the most brilliant colors. If the film be turned 

 round, still keeping it perpendicular to the polarized ray, the 

 colors will become less or more bright, and two positions will 

 be found, at right angles with each other, wherein no colors 

 at all are perceived. If the analyzer be turned round, the film 

 retaining its position, complementary colors will alternate, 

 together with points of invisibility, during each revolution. 



The colors of the film vary with its thickness, so that by 

 making grooves or lines of various depths, the most beautiful 

 patterns may be produced. Drawings of figures and land- 

 scapes are thus executed, and being mounted between glasses 

 in Canada balsam, are invisible, or nearly so, till exposed to 

 polarized light, when they are seen distinctly, arrayed in most 

 gorgeous colors. 



Various crystals exhibit, round their axes of double refrac- 

 tion, beautiful systems of colored rings, often intersected by 

 a black cross. Complementary colors may be produced in 

 them by turning round the analyzer. In large crystals, as 

 rhombs of Iceland spar, certain angles must be ground down 

 and polished in order to exhibit the rings. 



In those crystals having two axes of double refraction, a 

 double system of rin^ will be seen. A transverse section of 

 a prism of nitre will exhibit this phenomenon. 



The great advantage of employing the microscope in viewing 

 the colors of crystals, &c., by polarized light, arises from the 

 fact that, when crystallized on a slip of glass, many of the 

 small crystals will be arranged with their axes of double re- 

 fraction in the direction of the polarized beam. All such, 



17 



