POLARISED LIGHT. 113 



' Experiment 2. Screw the Nicol's prism into its place under the 

 stage, still retaining the double refractor over the eye-piece ; then, by 

 examining the object, there will appear in some positions two, but in 

 others only one image ; and it will be observed, that at 90° from the 

 latter position this ray will be cut off, and that which was first observed 

 will become visible ; at 180°, or one-half the circle, an alternate change 

 will take place ; at 270° another change ; and at 360°, or the com- 

 pletion of the circle, the original appearance. 



Before proceeding to the next experiment, it will be as well to 

 observe the position of the Nicol's prism, which should be adjusted 

 with its angles parallel to the square parts of the stage. In order to 

 secure the greatest brilliancy in the experiment, the proper relative 

 position of the selenite may be determined by noticing the natural 

 flaws in the film, which will be observed to run parallel with each 

 other ; these flaws should be adjusted at about 46° from the square 

 parts of the stage, to obtain the greatest amount of depolarisation. 



Experiment 3. If we now take the plate of selenite thus pre- 

 pared, and place it under the piece of brass on the stage, we shall see, 

 instead of the alternate black and white images, two coloured images 

 composed of the constituents of white light, which will alternately 

 change by revolving the eye-piece at every quarter of the circle ; then, 

 by passing along the brass, the images will overlap; and at the point 

 at which they do so, white light will be produced. If, by accident, 

 the prism is placed at an angle of 45° from the square part of the 

 stage, no particular colour will be perceived ; and it will then illustrate 

 the phenomena of the neutral axis of the selenite, because when placed 

 in that relative position no depolarisation takes place. The phenomena 

 of polarised light may be further illustrated by the addition of a second 

 double refractor, and a film of selenite adapted betweeo the double 

 refractors. The systems of coloured rings in crystals cut perpendicu- 

 larly to the principal axis of the crystal are best seen by screwing the 

 Nicol's prism under the stage, and employing the lowest object-glass : 

 place the crystals over the eye-piece, and use a tourmaline as the ana- 

 lyser." 



It was long believed that all crystals had only one axis of double 

 refraction ; but Brewster found that the great body of crystals, which 

 are either formed by art, or which occur in the mineral kingdom, have 

 two axes of double refraction as well as of polarisation. 



Nitre crystallises in six-side'd prisms with angles of about 120°. 

 It has two axes of double refraction, along which a ray of light is not 

 divided into two. These axes are eacli inclined about 2i° to the axes 



