126 
ELEMENTARY CHEMICAL MICROSCOPY 
with the two front lenses will cling in place. The illuminating 
beam will pass through this drop in the focal plane of the objec¬ 
tive 0 . The image resulting upon focusing the microscope will 
appear to be two hazy triangles of light united at their apices by 
a more or less marked brighter thread or band. In this band 
are seen the diffraction disks due to the infinitely small particles 
in suspension. By means of the ocular diaphragm all of the 
hazy triangles are cut off and the connecting thread or band of 
light alone allowed to appear in the field of view. 
In this instrument particles as small as may be counted 
and still smaller particles discerned, while in the ultramicroscopes 
of the types previously employed particles smaller than 5//^ 
could not be counted nor could their presence be satisfactorily 
demonstrated. 
