94 Dr. J. Kerr's Electro-optic 



dim and not well defined, although they present all the essen- 

 tial features of the phenomenon clearly enough ; but against 

 a larger tension of twelve pounds to sixteen, the bands are 

 beautifully distinct, narrow, sharply defined, and very black. 

 Some elementary electro-optic measurements with this liquid 

 will be described afterwards ; in the meantime I shall merely 

 ask the reader to notice the large range of measurable optical 

 effect here obtained, and with such a small cell, from a ten- 

 sion of one pound or less in the fixed compensator up to a 

 tension of sixteen pounds. 



14. Benzol (C 6 H 6 ). — This liquid also had been already ex- 

 amined thoroughly in the old cell. When tried in the new 

 cell it acts as a very good insulator (10), and gives excellent 

 optical effects of the same kind as those of carbon disulphide 

 (9). It requires no measurements, and very little observation, 

 to show that this liquid is far inferior to the former in inten- 

 sity and range of effect. Benzol does give the extinction- 

 bands clearly ; but they are never so fine as in CS 2 ; and I 

 think that I have never seen the bands clearly separated 

 in benzol against a tension of more than two pounds in 

 the fixed compensator. Still the electro-optic action is very 

 fine, pure, and perfectly regular. The light restored from 

 pure extinction by electric action in the cell is always extin- 

 guished perfectly by compression of glass in a direction 

 parallel to the lines of force, and always strengthened by tension 

 in that direction. 



15. Toluol (C 7 H 8 ). — This liquid is very like benzol in its 

 more patent physical properties ; like benzol also, it is a very 

 good insulator, and gives a good optical effect of the same 

 kind as CS 2 under electric action. In the only two careful 

 trials that I have given to this liquid, I found it particularly 

 difficult to obtain a pure initial extinction ; and although this 

 may have been caused by some unnoticed and accidental de- 

 rangements of the solid parts of the apparatus, I suspect it 

 was rather due to some faint specific action of the liquid. In 

 other respects I could not observe any clear difference between 

 toluol and benzol, the effects being equally regular and pure, 

 of exactly the same kind, and of much the same intensity and 

 range. The light restored from good extinction in the polari- 

 scope by electric action in the cell was always extinguished 

 perfectly by horizontal compression or vertical tension of glass. 

 The extinction-bands also were clearly developed in toluol, as 

 in benzol, against a small tension in the fixed compensator. 



16. Xylol (C 8 H 10 ). — This liquid also is very like benzol in 

 odour and appearance ; and it acts very similarly in the plate 

 cell, both as an insulator and in electro-optic experiment. From 



