1884.] modifying the refraction of plane polarised light. 



175 



We may perhaps account for the slight heating of the soiled 

 surface from the fact that the coating of small dust particles 

 which covers it may form a series of nuclei round which the 

 moisture of the air readily condenses. On the freshly cleaned 

 surfaces there are no such nuclei, the moisture is not condensed 

 and the heating effects are not produced. 



The bearing of the results of the experiments on the theory of 

 reflexion must be left for discussion in a future paper. 



Postscript. Nov. 1884. The experimental results have been 

 confirmed by further experiments made during the present 

 month. 



(2) On some experiments on the measurement of the capacity of 

 a condenser. By L. R. Wilberforce, B.A. 



The method employed in the following experimental determi- 

 nation was the same as that used by J. J. Thomson in his measure- 

 ment of "v", (Phil. Trans, iii. 1883), and may be thus briefly 

 described. 



For the resistance in one arm of a Wheatstone's bridge there is 

 substituted a broken circuit, a condenser and a commutator which 

 connects the electrodes of the condenser alternately with the 

 broken ends of the circuit and with each other. 



The commutator is worked at such a rate that its periodic 

 time is small compared with the time of swing of the galvanometer 

 needle. A key is introduced into the galvanometer circuit, and 

 the resistances are adjusted so that the position of equilibrium of 

 the needle is the same when the circuit is open and when it 

 is closed. 



If this is the case, a relation will exist between the periodic 



