494 
Proceedings of Royal Society of Edinburgh. [sess. 
The experimental work of this subject has been greatly facilitated 
by accessories invented and added to the microscope by Mr George 
R,. Smith, of Bradford, about three years ago. The complete 
Fig. 8. 
Curve A shows the maximum values of the projection width as e 2 changes. 
,, B ,, minimum ,, ,, ,, 
,, C ,, mean ,, ,, ,, 
,, D ,, ratio of the maximum to the minimum. 
apparatus is shown in fig. 9. A frame is fixed in grooves under 
the stage of the microscope, and it can be moved to and fro by 
a rack and pinion. One end of the frame carries a bell crank lever 
neatly pivoted, the upright arm of which carries a jaw for securing 
