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 cale 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 cale 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 well 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 
A oB 
