138 PHYSICS. 
every single ray, incident in certain directions, into two. One of these’ 
rays will be refracted according to the usual principles of refraction, hence: 
called the ordinary ray; the other, or extraordinary ray, follows quite a 
different course through the medium. This latter ray is polarized. The 
experiment is easily performed by making a small dot of ink on paper, and 
laying a crystal of Iceland spar over it. Two images of the dot will be 
seen, much to the surprise of every one who observes the phenomenon for 
the first time. This property was first observed in crystals of carbonate of 
lime, or Iceland spar, hence sometimes called double-refracting spar; it is 
not confined, however, to this mineral substance, belonging generally to all 
crystals whose primitive form is neither a cube nor an octahedron. In all 
doubly-refracting bodies there is one, and in some, two or more directions, 
along which, objects, when viewed through them, appear single ; these are 
called the lines or axes of double refraction. When the extraordinary ray 
is refracted towards this axis, the crystal is said to be positive; when from 
it, negative. 
Doubly-refracting crystals are sometimes applied to telescopes to measure — 
the diameters or distances of objects. A telescope provided with such an 
apparatus is called a Rochon’s -micrometer, from its inventor. The prism is 
movable, and placed between the objective and ocular. Let c (pl. 21, fig. 71) 
be a convex lens, casting an image of a distant object on a screen at fm. 
By interposing a prism of the proper character (generally two equal prisms 
of rock crystal cemented together) between the image and the lens, the 
ordinary ray will form an image at fm. while that of the extraordinary ray 
will be at f’m'. The distance between these two images increases with 
that of the prism from the screen, and decreases with the approximation of 
the latter ; the prism may then be brought so near to the screen that then 
the edges of the two images shall be in contact, asin fig. 72. The same 
reasoning applies when the lens c¢ is the objective of a telescope, and the 
images are seen through its ocular. We shall then have the following 
h 
formula for the tangency of the images: tang. v= afi lane- é; where z 
represents the centre of the prism, e the angle fmz, v the angle fem, f the 
focal length of the objective, and h the distance of the prism from the 
image. Now the values of f and the angle of deviation, e, are constant; h 
also is measurable by means of a graduation attached to the outside of the 
telescope, consequently the angle v can be ascertained from the formula. 
This angle is equal to that at which the object appears without any 
telescope, or the apparent diameter: knowing this, therefore, either the 
actual diameter or the distance of the object can be found, the other being 
known. 
A remarkable phenomenon of polarization is found in the brilliant colors 
produced by interposing thin plates of various substances between the two 
mirrors of the polariscope (fig. 69). These colors and their brillancy have 
been found to depend both upon the situation of the laminee and the 
relative position of the polarizing mirrors. If, for instance, the colors 
produced he of greatest intensity when the planes of the mirrors are at 
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