528 Proceedings of Royal Society of Edinburgh. [sess. 
device of the preceding paper), and so placed that the division 
of the field is vertical ; D, a quartz plate giving a rotation of 90° 
to the lower half of the field ; E, a double-image prism of such 
angular strength that the upper and lower halves of the field are 
superposed in the eye of the observer ; and F, the usual telescope, 
focussed on the dividing edge of C. E acts as analyser for both 
the upper and lower halves of the field ; for their respective plans 
of polarisation, having been made mutually perpendicular by D, 
are symmetrically disposed to the two vibration-directions of the 
double-image prism respectively. Measurements are made by 
rotating, not E, but A. 
The advantage of this arrangement is that it produces a field of 
view, the illumination of which is twice that of the ordinary 
field. A much reduced half-shade angle can therefore be em- 
ployed, from which there results, of course, a corresponding 
increase in the accuracy of the instrument. 
(. Issued separately January 14 , 1907 .) 
