Intelligence and Miscellaneous Articles, 339 



cally fulfil the condition, that the sum of the resistances of the n 

 arcs is equal to the resistance of one arc, to have the same current ; 

 i. e. the former must be made smaller than the latter, on account 

 of practical construction reasons. 



We may therefore sum up as follows : — The economical solution 

 of the division of the electric light is theoretically quite possible, but 

 'practically difficult to obtain. The division can never become un- 

 limited ; but ingenious inventors may nevertheless solve the problem 

 practically. 



The attempt by inventors to solve the question is therefore per- 

 fectly legitimate. If their attempt cannot lead to a perfect solution, 

 they may nevertheless do so approximately, and by it tend towards 

 real progress in illumination, inasmuch as by their attempts the 

 electric light may probably become more and more a successful 

 general rival to gas, which at present the electric light certainly 

 is not. 



Before I conclude, I must briefly avert to a paper on the Electric 

 Light by Mr. W. H. Preece, published in the Philosophical Ma- 

 gazine for January 1879, in which the author believes that he 

 has demonstrated from dynamical considerations that the division 

 of the electric light is impossible. This it certainly is under the 

 conditions introduced by Mr. Preece, viz. that the resistance of 

 each voltaic arc, or each incandescent wire, is maintained constant. 

 But it is unfair to the electric light to introduce this condition, 

 especially as it does not at all represent the question at issue. 



"When a number of lights are connected in series, the resistance 

 of each must be diminished, and when a number of lights are joined 

 parallel, the resistance of each must be increased in proportion to 

 their number, so as to maintain the total external resistance con- 

 stant. If Mr. Preece will introduce this condition into his equation, 

 he will find that theoretically the division of the electric light is 

 quite possible, i. e. that, theoretically, however the lights be ar- 

 ranged, the unit of light will always be produced by the same 

 expenditure of energy. Inventors should not, therefore, be down- 

 hearted. On the other hand, investors in gas need not hasten to 

 get rid of their shares ; for there are many questions involving 

 practical difficulties which still remain to be solved ; but, at the 

 same time, gas companies should be aware that they have a for- 

 midable rival in the field, and bestir themselves to maintain the 

 lead they hold by improving their own means of illumination and 

 extending its application. 



THE TRUE THEORY OF FRESNEL S INTERFERENCE PHENOMENA. 

 BY H. F. WEBER. 

 When the interference-fringes produced by a biprism or Fresnel 

 mirror are examined, the effect observed in the focal plane of the 

 eyepiece is the same as that which would result from two spherical 

 waves emanating from the two images of the luminous source, and 

 bounded respectively by the rectangular apertures occupying the place 



