136 PHYSICS: 
and fifty-eight millionths ; for glass, one third of a millionth, and fifty. 
millionths. 
The iridescence of mother-of-pearl, and other surfaces, is explicable in a 
similar manner. All such surfaces are found to have very fine parallel 
striz or grooves impressed upon them, a cast of which may readily be taken 
by means of soft wax. In this case the wax itself will show signs of 
iridescence. The colors, therefore, are produced by the interference of 
the light reflected from the bottom of the groove, with that reflected from 
the top. 
The colors of thin plates, or the Newtonian rings, may also be exhibited 
by reflecting a narrow beam of solar light in a.dark room upon a screen ; 
the mirror used must be concave, and of glass, with its axis coincident 
with the direction of the ray. 
h. Of Polarization and the Double Refraction of Light. 
.A ray of light is said to be polarized when it does not, as in ordinary 
rays, possess the same properties in every direction, with respect to 
reflection and refraction. If, for instance, an ordinary ray, ab 
(pl. 21, fig. 100), falls at an angle of 35° 25’, upon a plane plate of glass, 
blackened at the back, it will in greater part be reflected in the direction 
be,.according to the usual law; this latter ray, bc, is now polarized. 
Should this ray fall. upon a second blackened plate, similar to the first, and 
parallel to it, it will be reflected a second time, and, indeed, in the same 
plane. If now the second plate be rotated about the ray bc, still retainmg 
the. same angle of incidence, the plane of reflection will be changed, and 
the intensity of the twice reflected ray will diminish with the increase of 
the angle between the two planes of reflection; when this amounts to 90° 
the intensity of the ray will be 0. 
When two glass mirrors of the kind just described are combined, so as 
readily to admit of experiment in polarization, they form a polarizing 
apparatus or polariscope. Pl. 21, fig. 69, represents an instrument of this 
kind, as given by Norremberg. Two uprights are fixed firmly in a heavy 
foot, and inclose towards their lower end a frame, B, turnmg between these 
on a horizontal axis. The frame carries a polarizing glass mirror. The 
mirror is usually fixed with its plane at an angle of 35° 25’ with the 
vertical. A ray, ab, incident at this angle upon the mirror is partly 
transmitted, partly reflected; the reflected ray (now become polarized) 
takes the direction bc, and striking a plane mirror at ¢, is reflected back in 
the same direction, passing through the uncovered mirror, B, to the upper 
part of the apparatus. This upper portion sustains a ring graduated to 
degrees. Inside of this ring turns a second ring, with two small posts, 
between which is placed a second mirror, also turning on a horizontal axis. 
This mirror is of glass, blackened at the back, and is called the 
analysing plate or mirror, the lower one being the reflecting. An index 
310. 
