THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



MAY 1883. 



XLIV. Optical Combinations of Crystalline Films, 

 By Lewis Weight*. 



[Plate VI.] 



THE object of the following experiments is to illustrate the 

 facility with which simple combinations of mica-films, 

 such as can be readily put together by any one with the aid 

 of Canada balsam dissolved in benzol, may be made to demon- 

 strate not only the simpler chromatic phenomena of polarized 

 light, but also the more beautiful and complicated appearances 

 encountered at a more advanced stage of study. The colours 

 obtainable from such mica-films are more delicate and intense 

 than the usual selenite preparations, because while in selenite 

 those films which produce the lower and more intense orders 

 of Newton's colours are so thin as to be split with difficulty, 

 in mica they can be obtained with the greatest facility. Some 

 of the preparations are also, as demonstrations, superior in 

 themselves. 



Let us take first the simplest case, of different retardations 

 produced by different thicknesses of crystal, counteracted or 

 not by opposite retardations caused by another crossed film. 

 It has been usual to demonstrate these by two selenite wedges, 

 rotating one over the other. A simpler and more effective 

 demonstration is given by two wedges, each built up of similar 

 mica-films superposed, and cemented together, like those on the 



* Communicated by the Physical Society. 



Phil. Mag. S. 5. Yol. 15. No. 95. May 1883. Z 



