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XXVI. On a repetition of Dr. Kerr's Experiments on a new 

 Relation between Electricity and Light. By J. E. H. Gor- 

 don, B.A., Gonville and Caius College, Cambridge* . 



I HAVE been for some months engaged on a repetition of 

 the experiments described by Dr. Kerr in the Philo- 

 sophical Magazine for November and December 1875. I 

 have as yet been absolutely unable to obtain Dr. Kerr's 

 result ; and I think, when I have described the precautions 

 taken, I shall be able to show that either Dr. Kerr's success 

 or my failure is due to some cause not yet noticed by either 

 of us ; in any case the cause of the difference in our results 

 will be worth looking for. 



The optical arrangements. 



The polarizing apparatus consisted of a Nicol's prism and 

 collimator, as described by me in my last paperf. The end of 

 the collimator which carried the prism projected through a 

 hole in a screen, which protected the observer from stray light. 

 The collimating-tube was fixed in cement to the brick ex- 

 perimenting-table . 



Monochromatic light was used, generally that of a powerful 

 paraffine lamp, coloured red by means of a test-tube containing 

 coloured liquid and hung in front of the prism. 



Sometimes a solar spectrum was thrown on a card, a slit in 

 which admitted one colour to the Nicol. 



The analyzing-apparatus. 



This, which was all mounted on a massive block of wood 

 cemented to the brick table, consisted : — 



1. Of a Babinet's compensator (kindly lent to me by Pro- 

 fessor Maxwell). 



2. Of a powerful microscope used with a two-inch power, 

 at the back of which a Nicol's prism was fixed. The objective 

 and prism revolved together in a collar. The microscope and 

 collimator were both horizontal. The diagonals of the Mcols 

 were respectively inclined at ± 45° to the horizon ; distance 

 from lens of collimator to compensator 2 ft. 6 in. 



The bands were very distinct. Their apparent distance 

 from centre to centre, as determined by a camera lucida on the 

 eyepiece, upon a sheet of paper 10 inches distant, was 1\ inch 

 for red light. For their observation the eyepiece of the micro- 

 scope was furnished with fine cross-wires. 



* Communicated by the Author. 



t Determination of Yerdct's Constant. Kcad before the Royal Society. 

 June 15, 1876. 



