THE MICROSCOPE. 
will be entirely dark, no rays being transmitted to the eye¬ 
glass. 
If tbe microscope, on the contrary, be moved to the other side 
of the graduated semicircle, the same appearances will be pro¬ 
duced, and when it assumes the position m! the field will be 
again half illuminated, and beyond that point it will be dark. 
The arc of the graduated semicircle, included between the two 
positions m m and m'm', will then be the measure of the angular 
aperture of the object-glass, since that arc will correspond with 
the greatest obliquity, at which rays diverging from the object to 
14 
