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Transactions of the Society. 
and the cork mounting holding the prisms. One beam passes 
directly from the lamp through one of the prisms, while the other 
beam is obtained from the same lamp by reflection from the mirror 
through the other prism. This latter method is desirable because 
of the ease and rapidity with which the relative intensity of the 
two beams may be controlled. 
The method which is commonly used to resolve structure of 
this type is to adjust the illumination so that one large main beam 
and two diffracted beams at approximately right angles to each 
other emerge from the objective at the same time. With this 
method, however, not all of the light of the central beam enters 
into the actual formation of the image, and an obscuring effect on 
the image will always be present due to this excess light. When 
several orders of spectra are present this effect is not noticeable, 
but as the intervals separating the structure approach the resolv- 
ing power of the lens in use, this obscuring effect becomes more 
and more pronounced. 
The described method of removing this excess light by permit- 
ting two beams to be separately polarized, allows images of 
maximum clearness to be formed, their perfection depending upon 
the quality of the objective employed. 
It is hoped that some part of this paper may be found both useful 
and instructive to workers with the compound microscope. 
I 
