ON ELECTRO-OPTICS. 



157 



Report of the Committee, consisting of Dr. John Kerr, Mr. E. T. 

 GrLAZEBROOK, Lord Kelvin, and Professor A. W. Eucker, on 

 Electro-optics. 



The Committee report that Dr. Kerr has continued his work on 

 electro-optics during the year. His results are given in the appendix to 

 this report. 



Appendix. 



On Dispersion in Double 'Refraction due to Electric Stress. 

 By Dr. John Kerr. 



The object of the experiments was to find whether the optical effect 

 of electric stress varies with wave-length. The only dielectric examined 

 was CSj, and the cell was that described in the ' Phil. Mag.' for 

 March, 1880. The two nicols at the ends of the cell were placed with 

 principal sections parallel, and at 45° to the lines of force. Immediately 

 after the second nicol came a spectroscope, which received no other light 

 than that which passed through the two nicols and the uniform part of 

 the electric field. The lunette of the spectroscope was focussed, and 

 directed so that one of Fraunhofer's lines passed through the intersection 

 of the wires, and the instrument was untouched until the end of the 

 measurement. 



When the electric force is applied, the spectrum darkens ; and as the 

 potential rises through a sufficient range, a black fringe passes gradually 

 over the spectrum, entering at the violet end, and leaving at the red. 

 This fringe we know to be the place of phase- difference tt, or of retardation- 

 difference ^\, of the two component rays. 



The one measurement made in each case was that of the potential at 

 which the black fringe was bisected by the above-mentioned Fraunhofer's 

 line. The following table contains the results of the two sets of measure- 

 ments last taken. In each of the last three divisions of the table the first 

 two lines of numbers correspond to the two sets of measurements, and the 

 third line to their averages. The quantities represented by the numbers 

 are indicated in the first column, V standing for potential, Q for corre- 

 sponding optical effect, and Q' for the value of Q when V=180.^ 



The values of Q' are calculated from those of Q by the law of squares, this law 

 being verified for the three monochromatic lights G, F, D with the present apparatus 

 and arrangements, and through a range of effect equal to K. 



