ON THE POLARIZATION OF LIGHT. 



241 



passes, it experiences no double refraction; this is termed the 

 neutral axis, and no colour will be produced around it when 

 polarized light passes through the crystal in the direction of 

 this axis. 



Some persons, at the suggestion of Sir David Brewster, 

 employ a rhomb of Iceland spar for an analyzer, instead of a 

 Nicol's prism ; this should be so thick (when the selenlte is 

 placed on the stage) as to separate the complementary colours 

 by double refraction; and in order to protect the rhomb 

 from scratches, a plate of thin glass should be cemented to 

 two of its surfaces ; with this prism as an analyzer, and with 

 one of Nicol's as a polarizer, a film of selenite of uniform 

 thickness,^ and with a brass plate, three inches by one, per- 

 forated with a series of small holes, from the one-sixteenth to 

 the one-fourth of an inch in diameter, a variety of interesting 

 phenomena may be exhibited, as described by Mr. M. S. Legg, 

 in the Transactions of the Microscopical Society* If the brass 

 plate be placed on the stage, with the smallest hole in the field 

 of view, and an inch object-glass be employed as the magnifier, 

 the rhomb, when placed over the eye-piece, will give two 



images of the aper- 

 ture, as in fig. 154, 

 a. If the eye-piece 

 be now turned, the 

 images will describe a 

 circle; and if a larger 

 aperture be brought 

 into the field of view, 



the images will over- 

 Fig. 154. , \ ■ A 

 " lap, as shown in hg. 



154, b and c. If the Nicol's prism be now adapted to the under 



surface of the stage, and the eye-piece revolved, it will be found 



that in certain parts of the revolution there will be two images, 



whilst in others there will be only one, fig. 155, ab cd. If now 



a film of selenite of a certain thickness be placed underneath the 



brass plate, the apertures will be coloured red and green, fig, 



154, a' V d, and if the eye-piece be revolved, at every quarter 



* Vol. ii., part ii. 

 16 



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