LIGHT. 119 



grinding off its ends, and sliced lengthwise by a plane nearly, but 

 not quite, perpendicular to the principal plane. The parts into 

 which the prism is thus divided are joined in reverse positions, 

 and a diaphragm with a circular opening is placed at each end. 

 The light which passes through both diaphragms produces a 

 circular field divided by a diametral slit into two parts, in which 

 the planes of polarisation are slightly inclined to one another. If 

 the light which has been previously plane polarised be transmitted 

 it will be extinguished in the two parts of the field which lie close 

 together, and the light will become uniform in a position midway 

 between these. This position determines the position of the 

 original plane of polarisation with great precision. 



The light from certain parts of the sky is, as is well known, 

 polarised ; and the plane of polarisation depends upon the posi- 

 tion of the sun. Based upon this fact the late Sir Charles Wheat- 

 stone constructed a polar clock, in which the hour angle of the 

 sun, and consequently the local time, is approximately determined 

 by observing the position of this plane of polarisation. 



The interference of rays of polarised light for all differences of 

 path was first made the subject of direct experiment by MM. 

 Foucault and Fizeau, who ^showed the bands occurring in the 

 spectrum of such light after its passage through plates of crystal. 

 The same method may be employed for explaining the phe- 

 nomena of colour when the plates are thin ; and remarkable 

 forms of interference curves are produced in the spectra when 

 crystal plates of varying thickness are introduced. Apparatus 

 combining the spectroscope and polariscope for these experiments 

 will be exhibited. A polariscope, with a rapidly revolving double 

 image prism as analyser, was invented, independently and almost 

 at the same time, by Professor Mach in Vienna, and Mr. Spottis- 

 woode in London, for showing simultaneously, instead of succes- 

 sively, all the phases of rotatory and other polarisation. The 

 principle can be applied to eye observations and to projection. 



A compound analyser is also exhibited. This consists of two or 



