208 REPORT — 1887. 



(3) Tbat the resolutions of the Pai'is Congress with respect to the 

 ampere, the volt, the coulomb, and the farad be adopted. 



(4) That the resistance standards belonging to the Committee of the 

 British Association on electrical standards now deposited at the Cavendish 

 Laboratory at Cambridge be accepted as the English legal standards 

 conformable to the adopted definition of the Paris Congress. 



In reply, therefore, to the letter of the Board of Trade, the Secretary 

 forwarded a copy of the above resolutions, with a statement of some of 

 the reasons which had led to their adoption by the Committee. 



During the year the original standards of the Association have again 

 been compared by the Secretary. An account of this compai-ison and 

 of the very complete one made in the years 1879-80-81 by Dr. Fleming, 

 the details of which have not been published previously, will be given 

 shortly. 



At the last meeting of the Committee it was resolved, on the motion 

 of Mr. "W". H. Preece, seconded by Sir William Thomson, to recommend 

 the adoption of the Watt as the unit of power. 



The Watt is defined to be the work done per second by the ampere 

 passing between two points between which the difference of electrical 

 potential is one volt. 



The Committee were also of opinion that it is highly desii-able to 

 proceed with the construction of an air condenser as a standard of 

 capacity, and for this purpose they desire to be reappointed, with the 

 addition of the name of Mr. Thomas Gray and a grant of lOOZ. 



Supplement to a Report on Optical Theories. 

 By R. T. Glazebkook, M.A., F.R.S. 



In my Report on Optical Theories (' B. A. Report,' 1885) I gave an 

 account of Dr. Voigt's Theory of Optics. A recent communication of 

 his to Wiedemann's ' Annalen ' shows me that in one point I have unin- 

 tentionally misrepresented his views. 



As I understood his previous papei's, the quantities represented by 

 A, B, C (Wied. ' Ann.' xix. p. 874 ; ' Report,' p. 231), &c., are intended 

 to express completely, so far as the problem before us is concerned, the 

 action of the matter on the ether in the element of volume considered, 

 and that in all cases, even when one face of the element is on the 

 surface. 



I took the statement (p. 876) ' indem man die Wirkungssphare der 

 Molecnlarki-afte gegen die Grosse des betrachteten Volumen Elementes 

 soklein annimmt dass man die Wirkung die der Aether in demselben erfahrt 

 als nur von der Materie desselben Elementes herrlihrend betrachtenkann,' 

 which is precise and definite, as true always, and supposed that the forces 

 acting on the element were known up to the boundary. This being the 

 case, the surface conditions are X + A=X' + A', and not those implied in 

 Kirchhoff 's principle. Professor Voigt has explained that this was not 

 his meaning. When one face of the element is on the surface, the forces 

 acting are no longer known. The force denoted by A is to be taken as 

 made up of two — A„j and A^i^ in his notation — of which Aj^jj arises from 

 the action of the matter in the second medium, and all that is known is 

 that neither loss nor gain of energy is caused by such forces. This, it is 



