354 Proceedings of Royal Society of Edinburgh. [sess. 
case of the form of this instrument which is depicted in fig. 5. 
In the case of the form which is depicted in fig. 4 the sliding 
pieces A and B of the screen (fig. 7) are first set respectively to 
the two neighbouring strips of the spectrum whose intensity it is 
desired to compare, and then the lens-halves L and 1/ (fig. 3) are 
moved sideways normal to the plane of the paper to bring the two 
images of these strips one above the other in the plane SS'. The 
perfect contact of the edges is secured by moving the lens-halves 
vertically either nearer together or further apart, as has already 
been explained. 
(Issued separately November 5, 1904 .) 
