of Crystalline Films. 307 



with the same number of micas crossed in exactly the same 

 way, but of different thicknesses ; the phenomena appear actu- 

 ally reversed in character. Having seen this, we abandon 

 simply crossed films, and the following will be composed of 

 films superposed at angles of both 90° and 45°. Here another 

 cause of variety comes into play, since all films whose thickness 

 contains an odd \\, when superposed at 45° will circularly 

 polarize the light. Moreover we also know that if two such 

 films are superposed at an angle of 45°, the effect is to rotate 

 the plane of polarization itself (as shown for instance by the 

 rotation of a calcite cross) 45° from the original plane. Hence 

 the variety and scope for combination here are endless, the 

 phenomena always being beautiful; but I must only show you 

 a very few of such preparations. The first four are all com- 

 posed of films -J-X in thickness, and each contains the same 

 number of twelve films, and the lines show the successive 

 positions of the mica " axis." In the first they are 



Thus all the diagonal axes lie the same way. Now this second 

 preparation has the very same individual films differently 

 placed, thus: — 



You see the total difference in effect produced by the differ- 

 ence in crossing. 



The next one is thus arranged: — 



1 1 - 1 1 - I = I - 



This is an interesting combination, because the wave-decom- 

 positions indicate that the light should be nearly extinguished 

 when the Nicols are crossed, not only in the original black 

 cross, but also along the diagonals between. You see that it 

 is so ; but this result is still more completely brought out by 

 the next preparation, 



I - I - I — l - I - I 



where we get a nearly perfectly black square crossed by nearly 

 black diagonals as well as by the black cross. The next set of 

 five are all built of films one wave in thickness, as follows : — 



No. 1. 



8 micas 



1 - 1 x | - | 



2. 



10 „ 



+ + x + + 



3. 



12 „ 



+ + X X + + 



4. 



8 „ 



+ X + X 



5. 



8 „ 



1 / ■ — \ 1 / - \ (i. e. 

 successively rotated 45°) 



