58 Messrs. Ayrton, Mather, and Sumpner 



This arrangement is not an obvious one, as I can discover no 

 physical property common to one such series alone which 

 would lead us to make the proper selection. 

 The Table of wave-numbers so grouped is : — 



I. II. III. IV. V. 



2010-05 



2151-28 



2117-48 2295-53 



2077-32 2264-65 2442-60 



2033-72 2227-37 241476 2592-76 



2186-41 2380-47 2567-89 2745'74 



2143-00 2342-39 2536-43 2723-79 2901-75 



2301-81 2501-25 2695-31 2882-61 3060-44 



2463-66 2663-01 2857'02 3044-325 3221-86 



2628-22 2827-65 3021-70 3208-83 3386-38 



2795-76 2995-27 318908 3376-21 355347 



2966-30 3165-66 3359-42 3546-48 



In each of these series the first differences are the same 

 nearly, so they are " identical ; " and the second differences 

 are constant to a certain degree. 



Physical Laboratory, Johns Hopkins 

 University, April 1890. 



VI. Galvanometers. By Prof. W. E. Ayrton, F.R.S., 

 T. Mather, and W. E. Sumpner, D.Sc* 



IN order that a number of students may be able to work at 

 the same time in the Physical Laboratories of the City 

 and Guilds of London Central Institution we have en- 

 deavoured, as far as possible, to arrange the apparatus so that 

 each of the many experiments should be complete in itself. 

 This has led to the necessity of our constructing, and having 

 constructed for us, a large number of galvanometers of various 

 types, and from the specimens that are on the table it will be 

 seen that several of the galvanometers contain points of novelty. 

 A long series of comparative tests has been carried out with 

 all the more important types that are in our laboratories, and 

 we have therefore thought that a record of the results ob- 

 tained by the students, and by ourselves, will be of value in 

 affording data to guide others in the selection of instruments 

 most suitable for the objects in view, as well as in suggesting 

 future improvements in the manufacture of galvanometers. 



I. Astatic or Non- A static. 



One of the first questions that arises is whether it is desirable 

 to employ the astatic type in the construction of sensitive 

 * Communicated by the Physical Society : read January 17, 1890. 



