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In the former case the central rings inside the focus will appear faint and the outer ones unduly strong, while outside the focus the central rings will be too bright and the outer ones too feeble. But if the aberration is overcorrected the central rings will be overbright inside the focus and abnormally faint outside the focus.
Any irregularity in the shape of the rings which persists after the lenses have been accurately adjusted and the telescope has properly cooled may be ascribed to imperfections, such as veins or spots of unequal density in the glass forming the objective. The spherical aberration of an object glass may be undercorrected or overcorrected.
Fast acceleration snapped him back against the seat. The jet began to wander a little and he corrected automatically, and almost overcorrected! With infinite care he straightened out again, just as the plane was air-borne. Eyes riveted on the horizon, he felt for the switch that pulled up the landing gear and felt the plane spurt ahead as the drag of wheels and struts was removed.
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