cutting rock crystal for micrometers. 131 
shall be at right angles to each other. And it may farther 
be remarked, that even then, unless the pencil of light pass 
truly in the common plane of refraction of the wedges, four 
images will be formed, so as to destroy the effect of the 
combination. 
From tbe Press of 
W. BULMER & Co. 
Cleveland-row, St. James’s, 
London. 
