﻿Interference Colours of Chlorate of Potash Crystals. 67 



obtainable as an infinite series for all values of in the 

 space 0>6>a. 

 The series is 



V= ^.X sin 57r <9 sin— & J S7r (W) K 8 „(mr), . (16) 

 a. a. a — — v 



a a 



when r>r\ and for the case when r<r' it is only necessary 

 to interchange these quantities. 



A solution of the same problem is given by Macdonald in 

 his book on Electric Waves, which depends on a theorem in 

 an earlier portion of his book. That solution is in agreement 

 with the above. 



The method can be applied to three-dimensional problems, 

 and some interesting results are being obtained which I hope 

 soon to send in. 



X. Interference Colours of Chlorate of Potash Crystals 

 and a New Method of Isolating Heat Waves. By R. W. 

 Wood, Professor of Experimental Physics, Johns Hopkins 



University*, 



[Plate I.] 



THE remarkable colours exhibited by certain twinned 

 crystals of chlorate of potash have been studied bv 

 Stokes and Lord Rayleigh. 



The crystals are easily prepared by making a hot saturated 

 solution of the potash, and allowing it to cool slowly in a 

 large flat-bottomed tray. On breaking up the crystalline 

 mass, and shaking the tray gently in front of a window, 

 numerous highly coloured flakes will appear, which can be 

 lifted out by means of a small bent spatula, made of thin 

 sheet copper or brass. They should be dried on filter-paper, 

 and mounted in balsam, preferably between two glass prisms 

 of about ten degrees angle. The thin flakes have the re- 

 markable property of reflecting practically all of the incident 

 light of a certain colour or colours, while freely transmitting 

 the remainder of the spectrum. The reflected colours are of 

 great spectral purity, the width of the band being sometimes 

 not greater than the distance between the yellow mercury 

 lines. Seen by transmitted light the flakes often appear 

 delicately tinted, and the spectrum of the transmitted light 

 will be found to be crossed by one or more narrow and 

 intensely black bands. 



* Communicated by the Author. 

 F 2 



