influenced by the Earth' 's Motion ? 217 



being chosen. It was actuated by a Ruhmkorff coil and four 

 Grove cells, situated at some distance away. 



The various parts, all mounted upon the pivoted board, will 

 now be specified in order. First came the helium tube with 

 capillary vertical, then at a distance o£ 25 cm. a collimating 

 spectacle-lens, followed by the polarizing nicol. The field of 

 view presented by this nicol was contracted to a circular 

 aperture 7 mm. in diameter, and was further divided into 

 two parts by a " sugar-cell." This cell was the same as that 

 formerly used in a cognate research on the rotation of the 

 plane of polarization in bisulphide of carbon under magnetic 

 force *. " The polarimeter employed is on the principle of 

 Laurent, but according to a suggestion of Poynting (Phil. 

 Mag. July 1880) the half-wave plate of quartz is replaced 

 by a cell containing syrup, so arranged that the two halves 

 of the field of view are subjected to small rotations differing 

 by about 2°. The difference of thickness necessary is best 

 obtained by introducing into the cell a piece of thick glass, the 

 upper edge of which divides the field into two parts. The 

 upper half of the field is then rotated by a thickness of syrup 

 equal to the entire width of the cell (say ^ inch), but in the 

 lower half of the field part of the thickness of syrup is replaced 

 by glass, and the rotation is correspondingly less. With a 

 pretty strong syrup a difference of 2° may be obtained with a 

 glass j s g inch [inch =2*54 cm.] thick. For the best results 

 the operating boundary should be a true plane perpendicular 

 to the face. The pieces used by me, however, were not 

 worked, being simply cut with a diamond from thick plate 

 glass ; and there was usually no difficulty in finding a part 

 of the edge sufficiently flat for the purpose, i. e. capable of 

 exhibiting a field of view sharply divided into two parts .... 

 By this use of sugar, half-shade polarimeters may be made of 

 large dimensions at short notice and at very little cost. The 

 syrup should be filtered (hot) through paper, and the cell 

 must be closed to prevent evaporation." 



The light next traversed the quartz crystals, each mounted 

 upon a small stand admitting of adjustment in azimuth and 

 level so as to bring the optical axis into parallelism with the 

 line of vision. The analysing nicol, mounted near the end of 

 the board, was distant 102 cm. from the polarizer. After 

 passing the nicol the light traversed in succession a direct- 

 vision prism of sufficient aperture and a small opera-glass 

 focussed upon the sugar-cell. The aperture limiting the field 

 had been so chosen that, as seen through the spectroscope, the 



* Phil. Trans, clxxvi. p. 343 (1885) ; < Scientific Papers,' vol. ii. p. 363. 



