150 MALUS. 



ordinary, some the extraordinary, refraction, and an 

 equal number of each. But this hypothesis is radically 

 subverted by a very simple experiment. 



If we cause the second crystal to turn through one 

 fourth of a revolution round itself, retaining the paral- 

 lelism of its upper and under surfaces to those of the 

 first, the ordinary ray will now become extraordinary, 

 and the extraordinary will now undergo only the ordi- 

 nary refraction.* 



* The subject of double refraction, of which the most characteristic 

 results are here stated by the author, is one which is rarely made in- 

 telligible to a general reader by a mere cursory description, and with- 

 out going into some detail of the successive changes which result on 

 receiving the two rays emitted from one crystal of calc spar on to 

 another placed in successively varied positions with respect to the 

 first. Perhaps few points are however easier to exhibit experimen- 

 tally which affords by far the Readiest way of familiarizing ourselves 

 with the whole phenomenon and its laws. It is only necessary to 

 procure two moderately clear rhombs of calc spar, and attach to the 

 side of one of them a card containing a small hole at the centre. It is 

 then easy to look through the two crystals at the light admitted 

 through the small hole; and keeping the two crystals with their sur- 

 faces in contact, the one next the eye can be turned round so that its 

 angles point in different directions with respect to those of the other. 

 For this purpose, by far the most convenient arrangement is to fix the 

 two crystals in small tubes (such as card pill-boxes), which can turn 

 one in the other: and if the crystals, and consequently the hole, be 

 small (for the images not to overlap), it is very convenient to magnify 

 the images by a small lens fixed in the tube next the eye, so that the 

 object to be viewed in focus is the small hole at the farthest surface 

 of the second crystal. The series of changes are these: setting out 

 from a position in which the two rhombs are similarly situated, (as if 

 parts of one larger crystal,) there are two images Avell separated. 

 These are represented at B in the figure (the two at A being drawn 

 for comparison when only one rhomb is used). Now, making one 



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