Ub 



The Bujlit Hon. Lord Rcujh'iijh 



[April 12, 



be polarised in or perpendicularly to the plane of incidence, the 

 reflected light is polarised in the opposite manner. [Shown.] 



Similar phenomena, except that the reflection is white, are exhibited 

 by crystals prepared in a manner described by Madan. If the crystal 

 be treated beyond a certain point the peculiar reflection disappears, 

 but returns upon cooling. [Shown.] 



In all these cases there can be little doubt that the reflection takes 

 place at twin surfaces, the theory of such reflection* reproducing 

 with remarkable exactness most of the features above described. In 

 order to explain the vigour and purity of the colour reflected in 

 certain crystals, it is necessary to suppose that there are a considerable 

 number of twin surfaces disposed at approximate equal intervals. At 

 each angle of incidence there would be a particular wave length for 

 which the phases of the several reflections are in agreement. The 

 selection of light of a particular wave length would thus take place 

 upon the same principle as in diffraction spectra, and might reach a 

 liigh degree of perfection. 



In illustration of this explanation an acoustical analogue is 

 exhibited. The successive twin planes are imitated by parallel and 

 equidistant discs of muslin (Figs. 1 and 2j stretched upon brass rings 



BIRO CALL 



©sensitive: flame 



Detail of Lazy -tongs 



and mounted (with the aid of three lazy-tongs arrangements), so that 

 there is but one degree of freedom to move, and that of such a character 

 as to vary the interval between the discs without disturbing their 

 equidistance and parallelism. 



The source of sound is a bird-call, giving a pure tone of high 

 pitch (inaudible), and the percipient is a high pressure flame issuing 

 from a burner so oriented that the direct waves are without influence 



' Phil. Mag.' Sept. 188S. 



