Observations on various Liquids, 89 



double refraction, the slip is simply stretched along its length 

 by weights attached to its lower end. The arrangements for 

 this piece were somewhat troublesome, but not such as to re- 

 quire a particular description. There is no cement used in its 

 construction. The glass is attached to the stand above, and to 

 the weights below, by double bands of thin leather, which are 

 folded on the two ends of the slip, and kept adherent to the 

 glass by small blocks of wood and light clamping-pieces of 

 brass. The fixed compensator thus constructed acts very well 

 up to a tension of sixteen pounds ; and beyond this I have not 

 yet gone. 



6. Arrangement of the Pieces. — -The optical effects obtained 

 are sensibly modified by very small changes in the source of 

 light, and in the positions of the pieces ; but I shall not dwell 

 upon these variations here, as they were noticed at some length 

 in the second of my former papers. In all cases, the acting- 

 ray is horizontal, undeviated through its whole course, and 

 about a foot distant from the table which supports all the 

 pieces. Of the various arrangements tried, the following is, 

 I think, the best upon the whole. The diagram shows all the 

 pieces in horizontal section through the ray LM, L being the 

 source of light, and M the observer's eye. L is a flat paraffin- 



PC 



w 



j t=v-'m 



flame presented edgeways ; P is the polarizing Nicol ; A is the 

 dielectric cell (1), having its outer balls connected by copper 

 wires, one with the prime conductor and the other with earth ; 

 B is a couple of stationary compensating plates of glass (3), 

 hanging vertically, and mo anted so as to admit of the attach- 

 ment of stretching weights to one or the other or both (5) ; C 

 is a neutralizing plate which is sometimes required in mea- 

 surements and in the more delicate observations ; Q is the ana- 

 lyzing Nicol. The distance PQ is from 40 to 60 inches. The 

 hand compensator is not shown in the diagram ; when used, 

 it is held between the pieces C and Q. 



7. Conduct of an Observation. — -The pieces are brought most 

 conveniently into the line LM in a certain order of succession. 

 The piece first placed is the polarizer P : it is laid carefully 

 with its principal section at 45° to the horizon — that is, at 45° 

 to what is afterwards the axis of the electric field. The ana- 



