62 Transactions of the Royal Microscopical Society. 
in which is a corresponding pin-hole. The two pin-holes are so 
placed that a beam of parallel white sunlight (r) passing through 
both -will be perpendicular to the side of the glass prism on which 
it impinges. 
O 
o 
To use this apparatus it is adjusted in the sub-stage of the 
microscope, a drop of oil of cloves is placed on the upper face of 
the prism, the glass slide F F, on which the object is mounted in 
Canada balsam under the usual thin cover, G, is placed on the stage, 
