270 



Measurements oj Electrical Constants. [Mar. 28, 



In most insects the field of vision has a small region common to the 

 two eyes in the vicinity of the month ; it is chiefly developed in the 

 predatory species, and probably serves in determining the distance of 

 their prey from their mandibles. 



III. " Measurements of Electrical Constants. No. II. On the 

 Specific Inductive Capacities of Certain Dielectrics." By 

 J. E. H. Gordon, B.A. Camb. First Series. Commu- 

 nicated by Professor J. Clerk Maxwell, F.R.S. Received 

 March 9, 1878. 



(Abstract.) 



The author has, nnder Professor Clerk Maxwell's directions, carried 

 out some measurements of specific inductive capacities by a new 

 method. The essential features of it are : — 



(1.) It is a zero method. 



(2.) The electrified metal plates never fouch the dielectrics. 



(3.) No permanent strain is produced or charge communicated, as 

 the electrification is reversed some 12,000 times per second. 



The potentials of the electrified plates were about equal to that of 

 2,000 cells. 



The following are the results obtained : — The solid dielectrics were 

 plates 7 inches square, and from \ inch to 1 inch thick. 



Dielectric. 

 Ebonite, 4 slabs of 

 thickness, 

 s. i 1 li-noh 



4 5 2 5 2 5 4 J - Li< ^ 11 J 



(1.) 



(2.) 

 (3.) 

 about. 



Best quality gutta percha . 

 Chatterton's compound . . . 



f black 



1 vulcanised. 



Sulphur 



Shellac 



Specific Inductive Capacity. 

 1-5593} 



V^?>Mean .... 1-56215 



V5669) 



India-rubber 



1-5939 

 1-6080 

 1-5502 

 1-5988 

 1-6127 

 1-6362 



Solid paraffin, sp. 

 gr. -9109 atll°C. 

 Melting point 

 68° C 

 6 slabs, each j-inch 

 thick, about. 



(1.)* 

 (2.) 

 (3.) 

 (4.) 

 (5.) 

 1(6.) 



1-4986' 

 1-4943 

 1-4920 

 1-5033 

 1-4936 

 1-5034 



>Mean 1-49753 



* These results are corrected for cavities in the plates, 

 rected determinations is I "4864. 



The mean of the uncor- 



