346 Mr. T. H. Blakesley's Further 



we have in either case 



H 



But if we define V as the maximum value of 



-^, (3) 



we have 



K 



y= 



\/H 2 -K 2 

 We have therefore the relations 



V 2 = V, and Y 3 = 



vi-V 2 



It is quite immaterial which expression for V is assumed, 

 provided this be the quantity actually measured by experi- 

 ment. It has been found that the simple eye-estimates of the 

 distinctness of the fringes coincide fairly well with the 

 results given by the first expression. 



In a subsequent paper I hope to communicate some of the 

 more interesting results of experiments with the "wave- 

 comparer," giving the visibility-curves for the principal lines 

 of hydrogen, sodium, thallium, and mercury, with their pro- 

 bable interpretation by means of the preceding formulae. 



XXXIX. Further Contributions to Dynamometry, or the Mea- 

 surement of Power. By T. H. Blakesley, M.A., M.Inst. 

 C.E.* 



"1VT0W that the advantage of using the split dynamometer 

 JL l to measure power, as first proposed by me more than 

 half a decade since, seems to be at length receiving some 

 attention through the generalization of the method when 

 applied to transformers, discovered independently by Prof. W. 

 E. Ayrton and myself, it may be well to point out clearly the 

 principles upon which such an instrument must be inserted 

 into an electrical system to effect the measurement of a phy- 

 sical quantity, and the nature of the quantities which admit of 

 such measurement. 



In the first place, an exact idea must be formed of the 



* Communicated by the Physical Society : read February 27, 1891. 



