586 TRANSACTIONS OF SECTION G. 
black cross, and in rendering the optical effect independent of the position of the 
specimen in the field, we have effected by the device of substituting circularly- 
polarising apparatus for both analyser and polariser. To this end we take 
two thin plates of mica (or of quartz cut parallel to the axis of crystallisation), 
selected so as to give as nearly as possible a retardation of one quarter-wave 
length for yellow light (D-line) between their ordinary and extraordinary rays. 
For many purposes ordinary so-called ‘ quarter-wave plates’ will suffice: but 
the ordinary quarter-wave plates, as sold, differ considerably amongst them- 
selves ; and it is important that the pair chosen should be alike in optical pro- 
perty. One must be large, covering the whole field of view ; the other may be 
quite small. The larger quarter-wave plate is placed between the polariser 
and the object, and is preferably attached to the polariser. It is set with its 
own axis at 45° to the plane of polarisation of the polariser. Thus set it trans- 
mutes the plane-polarised beam into a circular-polarised beam. The cheirality 
of this beam is right-handed or left-handed according to whether the axis is 
set at 45° to the left or at 45° to the right. The smaller quarter-wave is placed 
between the object and the analyser, and is preferably attached to the front 
of the analysing Nicol, to rotate with it, if the Nicol is turned. It also must be 
set with precision with its own axis at 45° to the plane of polarisation of the 
Nicol. If it receives a beam of circularly-polarised light it transmutes that 
beam into plane-polarised light, and this is either cut off or transmitted by the 
analyser, according to whether its own axis is at 45° to the left or to the right ; 
or, in other words, according as to whether the cheirality of the analysing 
combination of quarter-wave and Nicol is opposed to, or concordant with, the 
cheirality of the polarising combination. 
Tf the quarter-waves are so set as to give ‘ dark field,’ then there will still 
be dark field even if the analysing combination be rotated through any angle. 
Or if they are so set as to give ‘ bright field,’ then there will still be bright field 
for every position of the analysing combination. With this apparatus one cannot 
change, as in the ordinary polariscope, from ‘ dark field’ to ‘ bright field’ by 
turning the analyser through 90°. To do this requires that one or other of the 
quarter-waves must be rotated in its own plane through 90°, or be reversed 
face for face by turning through 180° around an axis parallel to the plane of 
polarisation of the Nicol with which it has been combined. 
With the above disposition of circular polariser and circular analyser, we 
get rid entirely of the black cross ; and, further, with the exception of certain 
slight changes of tint, the optical appearance of any polarising object placed in 
the apparatus remains invariable at whatever angle that object may be placed in 
the field. 
4. Second Report on Gaseous Explosions.—See Reports, p. 247. 
5. The Work of the British Association Committee on Gaseous 
Hzplosions.1 By Duaaup Currk, F'.R.S, 
WEDNESDAY, SEPTEMBER 1. 
The following Papers were read :— 
1. International Electrical Standardisation.t By Ormond HiGMAN. 
? Published in Yngineering. 
