Measuring Angular Aperture, By Professor B. Keith. 285 
suring agh, which is the balsam aperture, can be easily applied. If 
the shutter be now placed on the under surface of the glass cover, 
it is evident that, whether it be closed up more or less, it cannot 
affect in any way the measurement of agh, because every point it. 
the image at g is the apex of a cone with a h for the diameter of 
its base. 
Fig. 2. 
In the second figure, the glass cover is removed, and the effect 
of aberration becomes strikingly apparent. The rays now, instead 
of all crossing at g, aberr all the way from F to C, and for every 
point along that distance the objective has a different aperture. 
The point at which the light is maximum does not coincide with 
the point at which the aperture is maximum. The shutter can now 
be placed so as to reduce the aperture, liut can nowhere be placed 
so as to take it all in, except almost in contact with the objective 
and opened nearly as wide as its diameter. 
It thus appears that, if the objective is corrected for aberration, 
the shutter placed in the plane of its focus has no effect whatever. 
And if the objective is not corrected, the shutter can only shut out 
light too much affected by aberration to pass through the slit, which 
is the very light of most importance, and that upon which the max- 
imum aperture depends. 
The extreme air angle (in this case 170°) is very properly given 
by the makers as indicating the extreme capacity of the objective. 
It is the only easily measured quantity that will answer the purpose. 
X 2 
