On Measurements of Electromotive Force Sfc. 409 



wave-length curves are drawn. If we differentiate the 

 ordinary formula for a prism, 



_ sin ± (A-\-d) 



sm 



H 



A, 



with respect to A, which we now regard as a variable, we 

 have 



old _ n cos \ A 



dA~~cos \ (A + d) 



or dd=AdA. 



The values of A for the different lines of the spectrum are 

 readily computed from the table of deviations and refractive 

 indices. To find, then, the deviation of a line after any 

 repolishing of the prism, we have merely to multiply the 

 change of the angle by the approximate value of A taken 

 from the table, and we obtain the change in the deviation of 

 the line, and hence also the deviation required. Thus, if the 

 new angle is found on measurement to be 59° 57' 44", 

 dA= — 10", and the deviation of the F line (say) will have 

 been changed by 10" x 1*13= — 11"*3. That is, the deviation 

 of the F line is now 41° 5P 36". The reduction from the 

 new to the standard angle is of course the reverse of this ; 

 and that the use of the table saves much labour in redeter- 

 mining the constants of the prism will be understood when 

 it is added that ours has been entirely refigured and repolished 

 by the maker as many as ten times during the present year. 



LIY. On Measurements of tlie Electromotive Force of a Con- 

 stant Voltaic Cell with moving Plates. By A. P. Laurie, 

 B.Sc. Edinb., B.A. Cantab*' 



IN the last number of the Philosophical Magazine I 

 described some experiments made with the view of 

 determining the electromotive forces developed during the 

 combination of zinc and iodine in presence of water. 



Since obtaining the above results I have been engaged on 

 similar experiments with voltaic cells in which the zinc is 

 replaced by cadmium, the cell consisting of a cadmium plate, 

 and a platinum plate immersed in a solution of cadmium 

 iodide in which a little free iodine is dissolved. This work 

 not being as yet completed, I do not propose to describe it 

 here, but merely to give an account of a special set of experi- 



* Communicated bv the Author. 



