160 Dr. J. Kerr's Electro-optic 



as a nonconductor. The optical conditions were unfavourable, 

 and the effect of electric force could hardly be detected from 

 pure extinction; but when the hand-compensator was used 

 (as with amyl hydride formerly), the effect came out very 

 clearly — an increase of light in the case of tension parallel to 

 lines of force, and a decrease to sensible extinction in the case 

 of compression. 



Oil of Resin is interesting for its great density, -9989. It is 

 a distillate of common resin, redistilled from caustic potash, 

 and is now largely used in the jute-manufacture. My speci- 

 men was a very clean trade-sample, of a clear amber-colour, a 

 pretty strong photogyre, and a good insulator. Tried in the 

 plate cell as a nonconductor, it gave a strong positive effect. 

 As soon as the machine was set in motion, the light was 

 restored from a faint red or blue, which was the best extinc- 

 tion, to an intense and sensibly pure white; and the effect was 

 strengthened by tension parallel to lines of force, and weakened 

 to good extinction by compression. This result agrees with 

 my former observations on solid resin, where also the effects 

 were positive, though very much fainter. 



Paraffin fuses into a purely transparent liquid at about 

 50° C. Tried in the fusion-cell as a nonconductor, it gave a 

 strong regular effect. A quarter turn of the plate restored the 

 light well from extinction; and the effect was strengthened by 

 tension parallel to lines of force, and neutralized by compres- 

 sion. The sluggishness of rise and fall of optical effect was 

 very manifest. There was also some disturbance, produced by 

 movements in the liquid, when the machine was in motion. 



Naplithaline, C 10 H 8 , fuses at a moderate temperature into 

 a transparent and very volatile oil. Tried in the same way as 

 paraffin, it acted very similarly. There was a greater disturb- 

 ance by gross movements in the liquid; but the optical effect 

 was regular and perfectly distinct, conspiring with tension 

 parallel to lines of force, and neutralizing compression. 



5. Of this series, the eight members corresponding to the 

 following values of n were examined in order: — 

 16, 8, 5, 4, 3, 2, 1, 0. 

 The first six are purely negative, falling in power perceptibly 

 and regularly from first to sixth ; the seventh and eighth arc 

 purely but feebly positive. 



Cetyl alcohol (w = l 6) fuses into a transparent and nearlv 

 colourless plate, at or near 50° C. It was tried in the fusion- 

 cell as a nonconductor, after the initial extinction had been 



