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Chemistry and Physics. “is = wo 
ows :— 
(2.) The calc spar crystal is removed, and the Nicol’s prism exchanged 
for a doubly refracting rhomb of cale spar. When a round aperture is 
inserted at HE, two images of it are obtained in different colors; by rota- 
ting the rhomb the tints pass into one another. 
Crystals improperly termed dichroitic are now represented by using 
plates of glass of the same color, and weakening the intensity of one of the 
beams of light. 3 
(6.) If a crystal of cale spar cut at right angles to its axis is inserted 
between A and HE the phenomena are obtained which dichroitic crystals 
exhibit when they are used in a polarizing apparatus as an analyzing 
arrangement. 
striking experiment. A cube of clear-glass 2 inches in diameter was 
placed upon a colored glass cube 1 inch in diameter. When viewed with 
both eyes from above, the colored glass appeared to be converted into a 
four sided prism of double the height. (A piece of glass 1 inch thick with 
two parallel sides, placed on a cork $ inch in diameter shows this very 
€ceptive phenomenon quite well.) If print be viewed through a lar 
thomb of calc spar by both eyes it will be seen that the two images do 
___* Optische Studien, by Prof. H. W. Dove, Berlin, 1859. 
