Polarized Light through Double-refracting Crystals. 141 



the colours due to the compounding of the different parts of the 

 two spectra. 



The beauty of the effect depends of course greatly on the 

 extent to which the double-image prism separates the two images. 

 It should be so cut that the compound colours caused by the over- 

 lapping of the spectra shall be as different as possible from either 

 of their constituent colours. The selenite should then be set at 45°, 

 so as to make the spectra of equal and the bands of maximum 

 intensity. 



With films which give numerous bands the effect is very 

 beautiful, and may be still further enhanced by rotating the 

 polarizer, when the bands will shift their position, at the same time 

 changing their colours. 



Eosjyeriments ivith Sections of Bouhle-refracting Crystals giving 

 Coloured Rings. 



The coloured rings produced when polarized light is transmitted 

 through a double-refracting crystal cut perpendicularly to its axis, 

 have always been admitted to be among the most beautiful of the 

 phenomena which the science of optics can produce. 



When homogeneous hght is used it is well known that the rings 

 assume entirely the colour of the light used, the spaces between 

 the coloured rings being black. 



The splendour of the phenomena, however, obtained by the 

 use either of ordinary or of homogeneous light, is incomparably 

 inferior to that displayed by projecting the rings against the 

 spectrum. The spectrum microscope is admirably suited for this 

 exhibition. 



The method I adopted was simj)ly to place the section of the 

 crystal immediately over the eye lens of the microscope, and between 

 it and the analyzing prism. 



The rings are thus seen of every colour in the spectrum, alter- 

 nating with jet-black rings between each, those in the red being 

 the broadest, and the breadth of the rings gradually diminishing to 

 the most refrangible end of the spectrum. 



It is impossible to give any satisfactory idea of the appearances 

 by mere description, and no little skill or labour would be required 

 to paint any adequate representation of the effects seen in some of 

 the following combinations. 



Take, as an example, a section of a crystal of sugar which 

 gives a very fine system of rings. (Fig. 3, Plate XCV.) I have 

 counted easily as many as forty-five when projected against the 

 spectrum. This crystal is one of those which gives in polarized 

 light two black brushes, not a black cross like Iceland spar. When 

 the Nicol's prisms are at right angles the brushes are at their 



