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 rri m', the field will be 

 again half illuminated, and beyond 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 



