hy transmission through Fluids. 421 



they form excellent analysers to apply to the eye-pieces of 

 microscopes ; for experiments on circular polarization they 

 form very useful polarizers. The blue, yellow and hair- 

 brown tourmalines are perhaps superior when free from flaws, 

 but cannot be so readily obtained perfect. 



Agates polarize by dispersion, but are not suitable for ex- 

 periments on other substances. 



Iceland spar and quartz polarize light by double refraction. 

 The beam of light transmitted through them is divided into 

 two portions, O and X, polarized in opposite planes, but pro- 

 ceeding in paths so closely posited, that they may both be the 

 subject of observation at the same time. This is not the case 

 however in what are termed Nicol's prisms, which are pur- 

 posely so constructed as to allow only one portion to be trans- 

 mitted when held in one direction, whilst the other is trans- 

 mitted when the prism is revolved a right angle. The rhombs 

 of Iceland spar are usually rendered achromatic by cementing 

 a piece of glass properly adjusted to them. In order to render 

 quartz available a particular construction is necessarj', but 

 when well-made, double refracting analysers of quartz are 

 equally useful for these experiments. 



It does not matter by what means our polarized beam is 

 first obtained, or in what manner the same is analysed, or 

 whether one mode be used to polarize the light and another 

 to examine or analyse the same. In all cases the result will 

 be dependent on the relative positions of the polarizer and 

 analyser, and if these are placed in such a direction to each 

 other as that X is not transmitted to the observer's eye, and 

 therefore dark, then O will be transmitted ; but if either the 

 analyser or polarizer is changed to a position exactly at right 

 angles to its former direction, then O will be obscured, whilst 

 X becomes light. When one portion has obtained its greatest 

 obscuration, or is invisible, the other will have obtained its 

 maximum of illumination, whilst they pass gradually from one 

 condition to the other at the intermediate positions*. 



* A most convenient mode of obtaining polarized light which occurred 

 to me about two years ago, and which I have since shown to several friends, 

 consists in placing an ordinary looking-glass on the outside of a common 

 window, the bottom of the mirror being placed close to the lower portion 

 of one of the panes of glass, whilst the top is inclined from the window to- 

 wards the sky, so as to reflect the pane of glass into a horizontal position. 

 Standing then within the apartment, and looking through a tourmaline or 

 other analysing medium at the light of the sky reflected from the mirror 

 through the paneof glass, it will be found intensely polarized, and on intro- 

 ducing any substance, such, for instance, as a fluid contained in a glass 

 tube or a crystal, &c., between the analyser and the polarizer, it may be 

 readily examined. The great advantage of this mode of obtaining polar- 

 ized light consists not only in the perfection of the polarization, but also 



