Dr. North on the Angle of Aperture in Microscopes. 229 
to have more aberration. View of depth is therefore necessarily 
more limited. Still, for extremely slight elevations and depres- 
sions, and for very thin objects, some restriction will afford a more 
exquisite discrimination, provided the spherical correction is car- 
ried toa degree of perfection in due proportion to the angle. 
Indeed, whatever tends to bring the performance of an object- 
glass nearer to mathematical exactness, must limit its focus. Stop 
off aperture and we see farther in depth, because we see more in- 
distinctly in regard to minute details. Again, we may gain the 
ally as we carry the focus from the upper to the lower edge, the 
ellipticity of the figure is plainly seen to change, as evidently 
by the laws of geometry it must do, from the change of angular 
rection of its different points to the lens. Instead of the right 
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