218 Lord Kayleigh : Is Rotatory Polarization 



yellow image under observation was sufficiently separated 

 from the neighbouring red and green images corresponding- 

 to other spectral lines of helium. The position of the analysing 

 nicol was read with a vernier to tenths of a degree — an ac- 

 curacy which just sufficed, and the setting could be made by 

 causing the two halves of the field of view afforded by the 

 sugar-cell to appear equally dark. 



A good deal of time was spent in preliminary experiment 

 before the best procedure was hit upon. It is necessary 

 that the optic axes of the crystals be adjusted pretty accu- 

 rately to the line of vision, and this in several cases involved 

 considerable obliquity of the terminal faces. In these adjust- 

 ments the sugar-cell and its diaphragm are best dispensed 

 with, the crystals being turned until the rotation required to 

 darken the field is a minimum and the darkness itself 

 satisfactory. When the first crystal has been adjusted, a 

 second is introduced and adjusted in its turn, and so on. In 

 some cases a further shift of the crystal parallel to itself was 

 required in order to remove an imperfection from the part of 

 the field to be utilized. In the end a fairly satisfactory 

 darkness was attained, but decidedly inferior to that obtainable 

 when the quartzes were removed. Fart of the residual light 

 may have been due to want of adjustment ; but more seemed 

 to originate in imperfections in the quartzes themselves. 



In my former experiments upon bisulphide of carbon 

 advantage was found from a device for rocking the plane of 

 polarization through a small constant angle*. During the 

 observations now under discussion this effect was obtained by 

 the introduction of a second sugar-cell, not divided into two 

 parts or seen in focus, just in front of the analysing nicol. 

 The cell was mounted so that it could slide horizontally in 

 and out up to fixed stops. The thickness of the cell being- 

 sufficient, the strength of the syrup was adjusted to the 

 desired point. Thus when the nicol was correctly set, the 

 upper half of the field was just distinctly the brighter when 

 the cell was in, and the lower half with equal distinctness the 

 brighter when the cell was out, the object to be aimed at in 

 the setting of the nicol being the equality of these small 

 differences. For the results now to be given the setting of 

 the nicol was by myself and the reading of the vernier 

 was by Mr. Gordon. A second observer is a distinct 

 advantage. 



As a specimen, chosen at random, I will give in full all the 



* Loc. cit. ; ' Scientific Papers,' vol. ii. p. 366. 



