On the Configuration of the Scales of Butterjiie^ Wings. 279 



continually increase the tension, and when it became infi- 

 nitely great, the tension would be so too. But what is the 

 true action of a slender wire, connecting in this way a pair of 

 plates ? A certain amount of electricity passes along it, but 

 not the "whole quantity that the plates could generate in a 

 given time; yet we cannot but suppose that all that does pass 

 comes from the whole surface exposed, and not from a frac- 

 tional portion thereof. The water and zinc are ready to ge- 

 nerate, and as it were attempting to drive a fresh quantity of 

 electricity through the wire ; and accordingly, as the quantity 

 that actually passes becomes a greater and greater portion of 

 what the system actually tends to put in motion, the tension 

 becomes less and less. The tension would therefore become 

 zero, if the whole circle wires, plates, and electrolyte could 

 carry all that the zinc and water could generate. The limit 

 prescribed to its diminution is the conducting power of the 

 electrolyte, which is the worst conductor of the system. 



This hypothetical condition, of a tension ranging near zero, 

 is most nearly approximated to in a thermal pair. 



Suppose now that everything remains the same, as respects 

 wires, electrolyte, distance of plates, &c., except that the di- 

 mensions of both plates are doubled. Shall we increase the 

 tension ? No; for although the surface in action is doubled, 

 and the absolute quantity which the system could generate is 

 doubled, yet the quantity that passes both the primary and 



secondary wire is also doubled : the ratio — is therefore the 



a 



same as before. For this reason, increasing the magnitude of 

 the plates, increases the quantity only, and not the tension. 



Under all these circumstances, therefore, the tension de- 

 pends on the ratio of the quantity that does pass the combi- 

 nation, to the quantity that the system tends to put in mo- 

 tion. 



[To be continued.] 



XXXVII. On the Configuration of the Scales of Butterfiies' 

 Wings, as exhibited in the Microscope. By the Rev. Edward 

 Craig, M.A., F.ll.S.E. 



[With Figures : Plate I.] 



7b the Editors of the Philosophical Magazine and Journal, 

 Gentlemen, 



¥ F the following observations are not rendered unnecessary 

 *^ by earlier correspondents, they are at your service for in- 

 sertion in the Journal. 



