What is the chord length of the mire measured by the keratometer without an auxiliary lens?

The chord length of the mire measured by the keratometer without an auxiliary lens is typically in the range of 3.8 mm to 2.6 mm. This measurement is essential in determining the corneal curvature, as the keratometer uses mires to project reflected rings onto the cornea, which are then analyzed to calculate the curvature based on how these rings appear.

The setting of the keratometer without an auxiliary lens indicates that the measurement is being taken with standard optics, which influence the size of the mires observed. This smaller range aligns with the typical dimensions of the mire patterns measured under these conditions, reflecting the curvature of a regular cornea.

Understanding the context is important: the range of measurements given provides insight into the normal variability one might encounter when evaluating different eyes and different positions of the keratometer. Thus, the selection of this answer highlights an awareness of the instrument's functional range without additional lenses, showcasing a grasp of both the instrument's operation and the underlying principles of corneal shape measurement.