Observations on various Liquids. 91 



creasing continuously from zero. Horizontal tension is found 

 to strengthen the effect of electrical action in every case, while 

 horizontal compression, with similarly perfect distinctness and 

 regularity, weakens the effect down to sensibly pure extinction. 

 When the electric action is feeble, a certain small compression 

 of the compensator extinguishes the electrically restored light 

 as a whole, or simultaneously in all its parts ; but when the 

 action is intense, the axal part of the field requires a greater 

 compression than the outer parts, and the extinction-pheno- 

 mena take the form of two dark bands, which will be particu- 

 larly described immediately. 



When the plane of polarization of the light rendered by the 

 first Nicol is either horizontal or vertical, either parallel or 

 perpendicular to the lines of force, and the second Nicol is at 

 pure extinction, the effects of electrization are evanescent ; if 

 they do appear, they are irregular in character and trifling in 

 quantity. 



There is certainly no rotation of the plane of polarization in 

 the present case ; for when the light restored by electric force 

 is tried by small rotations of the second Nicol in contrary di- 

 rections from the position of initial extinction, it is found to 

 be similarly and equally affected by the two movements. 



The action of dielectrified carbon disulphide upon trans- 

 mitted light is therefore similar to that of glass extended in a 

 direction parallel to the lines of force : it is a sensibly pure case 

 of what is known in optics as a uniaxally birefringent action, 

 the axis being parallel to the lines of force, and the action 

 positive. Of two component vibrations, which are polarized 

 in planes respectively parallel and perpendicular to the lines 

 of force, the latter is relatively retarded. 



10. Carbon Disulphide as an Insulator. — To test the insula- 

 ting power of this and other liquids, I compare the striking- 

 distance of the prime conductor when the wire connecting it 

 with the cell is in position and out successively. When the 

 cell is charged with carbon disulphide the result is decisive. 

 All the pieces being placed as in the experiments just de- 

 scribed, the machine is worked at an ordinary rate, and sparks 

 are drawn from the prime conductor on the knuckle, or on a 

 metallic ball connected with earth ; the wire from prime con- 

 ductor to cell is then removed, and sparks are again drawn 

 from the prime conductor now simply insulated. 1 have seen 

 very little perceptible difference (if any) between the two 

 cases, the sparks being of much the same density and length 

 when the' connecting wire is in position and when it is out. 



I present this fact prominently, because it has an important 

 bearing on the interpretation of the electro-optic experiment 



