166 Prof. D. B. Brace on a 



be present under the slightest effect induced by the cement 

 or otherwise. Thin cover-glasses are carefully selected and 

 cemented on the brass caps or end-pieces which screw into 

 the tube. No moisture or acid used on the metal must be 

 allowed to reach the plate. The liquid itself shows no action 

 on spar. As it is quite volatile the cell is closed tight, with a 

 bubble of air remaining in to compensate for expansion. The 

 sensitive strip may be mounted in the same cell, but in order 

 to vary the sensibility one of the plates must be capable of a 

 slight displacement. The mechanical difficulties are greater 

 in such a system ; but higher efficiency is possible. On the 

 other hand, greater simplicity is obtained by mounting the strip 

 in a separate cell and then placing the two cells in a common 

 tube and rotating the first, as is usually done in the mounting of 

 the Lippich system. With such thin strips of spar, of course 

 care must be exercised in mounting them so that they will not 

 fracture or suffer strain. This was done by cutting away a slight 

 portion in the inner circumference of the caps of the tube and 

 cementing the ends of the strip to these with the cement 

 mentioned. Thin cover-glasses were then cemented over the 

 ends of the blackened brass cell and the liquid inserted through 

 a small opening which was later sealed up. However, such a 

 strip might at the outset show no trace of depolarization, and 

 later not give perfect blackness owing, manifestly, to the 

 setting of the cement and the induced strain in the strip. 

 This was obviated by cementing one end only and leaving 

 the other slightly free within retaining contacts. When these 

 precautions are taken and the system examined in the ordinary 

 way with analyser and telescope, the surprising sharpness 

 between the fields is at once observed and, with a neutral 

 setting, as perfect a vanishing line is obtained as with the 

 most delicately adjusted strip in the best spectrophotometer. 

 This system, properly mounted with parallel end-plates, when 

 placed in any ray system will give a single image and 

 cause no deviation of the axis of the ray system. The 

 sensibility of the system can be varied easily and a 

 vanishing line obtained. It may also be used for any 

 colour and the neutral setting will not vary for different 

 parts of the spectrum. When the liquid is freed from 

 suspended particles by filtration no diffused light emanates 

 from the polarizer and a perfectly black field is obtainable 

 even with direct sunlight. These conditions are not so 

 quickly obtained as with the system of Lippich, but when 

 they are, a greater sensibility is possible. For a single bundle 

 of rays or such as would emanate from a narrow source or 

 slit, it is evident that, on account of the narrow edge of the 



