86 



Exp. 3. If we now take the plate of selenite thus prepared, 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 (as in 

 fig. 7), composed of the constituents of white light, which will alter- 

 nately change (by revolving the eye-piece) at every quarter of the 

 circle ; then by passing the brass along, so as to bring the larger ori- 

 fices in succession into the field, the images will overlap, and where 

 they overlap white light will be produced (see fig. 5). If by accident 

 the prism should be placed at 45° from the position just indicated (see 

 fig. 12), no particular colour will be observed, and it will then illus- 

 trate the phenomenon of the neutral axis of the selenite, because 

 when placed in that relative position no depolarization takes place. 



The phenomena of polarized light may be farther illustrated by the 

 addition of a second double-refractor, and a film of selenite adapted 

 between the double-refractors. 



Exp. 4. By placing the apparatus as described in the first experi- 

 ment (that is, removing the Nicol's prism and plate of selenite, but 

 retaining the brass plate), we shall observe the two images as shown 

 in fig. 4 ; then by placing the second double-refractor over the first, 

 so as for all the faces of the one to be parallel to all the faces of the 

 other, as if they formed but one piece, the eye will perceive two dis- 

 tinct images, but at twice the original distance from each other (see 

 fig. 8). If we now turn the crystal nearest the eye, from left to right, 

 two faint images will appear ; continuing the turn, the four images 

 will be all equally luminous ; and when the crystal has turned round 

 90°, there will be only two images of equal brightness. Continuing 

 the turn, other two faint images will appear; further on the four 

 images will be equal ; still further they will be unequal ; and at 180° 

 of revolution they will all coalesce into one bright image. 



Exp. 5. The above results will be rendered more interesting by in- 

 terposing between the doubly-refracting crystals the film of selenite. 

 Place the doubly-refracting crystals and the selenite upon the brass 

 mountings, so that the marks upon the brass mountings (as in figs. 13, 

 14 and 15) shall correspond. Instead of the two white images, as in 

 the preceding experiment, we shall see Ihree, of which the two outer 

 ones will be one colour (say green), and the middle its complementary 

 colour or red ; by turning the crystal nearest the eye, the middle 

 image will gradually divide, until the completion of a quarter revolu- 

 tion, when four images will appear, of equal brilliancy, two of each 

 colour; revolve the crystal until the completion of the half circle, and 

 the three images will reappear, but with different properties, the outer 



