THE MICROSCOPE. 



will be entirely dark, no rays being transmitted to the eye- 

 glass. 



If the 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' m', the field will be 

 again half illuminated, and J>eyond that point it will be dark. 



Fig. 9. 



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 



