Some weapon aiming sights (e.g. FN FNC) require the user to align a peep sight (rear, nearby sight, i.e. which will be out of focus) with a tip (which should be focussed and overlaid on the target) at the end of the barrel.
Mount Airy | floppy disk | Mount Airy, North Carolina | Mount Airy, Maryland | Hard disk drive | hard disk drive | Golden Disk Awards | Universal Disk Format | Mount Airy, Philadelphia | disk | Mount Airy, Richmond County, Virginia | Floppy disk | Alderson disk | Hard Disk 20SC | Hard Disk 20 | George Biddell Airy | ESP-Disk | Double-sided disk | disk storage | Airy-0 | Poincaré disk model | Mount Airy, Georgia | "Mount Airy" | Macintosh External Disk Drive | Disk storage | Disk partitioning | Disk'O | disk (mathematics) | Disk II | Disk formatting |
Rayleigh defined the somewhat arbitrary "Rayleigh criterion" that two points whose angular separation is equal to the Airy disk radius to first null can be considered to be resolved.