286 Measuring Angular Aperture. By Professor B. Keith. 
It is further plain from the figure that if light about 1° outside of 
40° could pass through ab the maximum air angle would he 
180°. 
Objectives are constructed allowing light making an angle of 
several degrees outside of 41° to pass through the front, if water or 
balsam is interposed between the objective and the cover. It is 
evident that for such objectives something more than a maximum 
air angle of 180° is required to indicate their extreme aperture. 
To simply state the maximum air angle at 180° does not do them 
justice because the balsam angle corresponding to that air angle is 
only 82°, and the objectives alluded to have a balsam angle of 100° 
and upwards. 
Alexandria, Virginia. 
