﻿i Intelligence and Miscellaneous Articles. 401 



angle of 72° to the direction of these emergent rays. Each of these 

 rays will, in virtue of the principles enunciated above, be converted 

 by reflection into a circularly polarized ray ; but one will be a right- 

 handed, and the other a left-handed ray ; and the difference of phase 

 produced by the doubly-refracting plate will be undisturbed by the 

 reflection. This difference of phase depends, as is well known, upon 

 the wave-length — in other words, upon the colour of the light ; so 

 that the two circular rays will combine to form a plane-polarized ray, 

 whose plane of vibration depends upon the difference of phase, i. e. 

 upon its colour. And if, finally, the light be then examined by an 

 analyzer in the usual manner, we shall have all the phenomena of cir- 

 cular or successive polarization. 



From what has been stated above, it appears that the direction of 

 motion in the two circular rays, and consequently the order of colours 

 produced, depends upon the position (to the right or left of the plane 

 of incidence) of the ray which has been most retarded in its passage 

 through the crystal plate. If, therefore, the plate being in a given 

 position, the colours appear in an ascending order, then on turning 

 the plate through 90° in its own plane, or on turning it over about 

 an axis in the plane of incidence, the swifter and the slower rays will 

 change position, and the order of colours will be reversed. 



The reversal of the order of colour may be exhibited in another 

 way. Uniaxial crystals are divided into two classes : — one, called 

 positive (e. g. quartz), in which the extraordinary ray moves more 

 slowly than the ordinary ; the other, called negative (e. g. Iceland 

 spar), in which the ordinary ray is the slowest. If, therefore, a plate 

 of quartz placed with its axis at 45° on one side of the plane of in- 

 cidence give the colours in one order, a similar plate of Iceland spar 

 similarly placed will give them in the. reverse order. 



The same principles apply to the case of biaxial crystals cut parallel 

 to a plane containing the two optic axes. A ray of plane -polarized 

 light transmitted through such a plate is divided into two, whose 

 vibrations respectively bisect the angles formed by the two axes ; 

 the line which bisects the smallest angle is called the intermediate 

 section, and the line perpendicular to it the supplementary section ; 

 and the order of the colours depends upon the relative velocity of the 

 two rays. In selenite the ray whose vibrations lie in the supple- 

 mentary section is the slowest ; in mica it is the swiftest. Hence 

 these two crystals will, all other circumstances being alike, give 

 opposite orders of colour, and may be regarded as positive and ne- 

 gative respectively, like quartz and Iceland spar. 



The phenomena by which these principles may be illustrated are 

 very numerous and varied, but are better seen than described. 



LIV. Intelligence and Miscellaneous Articles. 



ON A BAROMETER WITHOUT MERCURY. BY A. HELLER, PRO- 

 FESSOR OE PHYSICS IN THE OBER-REALSCHULE IN OFEN. 



HHHE atmospheric pressure at a given time and place may be deter- 



•*- mined either by means of an apparatus which depends upon the 



principle of communicating- tubes, or by determining the specific 



