What is the pin on the outside of the keratometer designed for?

The pin on the outside of the keratometer is specifically designed to assist with proper alignment of the instrument with the outer canthus of the eye. This ensures that the measurement of the cornea is accurate by helping the practitioner position the keratometer correctly relative to the patient's eye. Proper alignment is crucial in obtaining precise readings, as any misalignment could lead to erroneous measurements of the corneal curvature.

The other options, while they may seem plausible, do not serve the intended purpose of the pin. For example, it does not hold a contact lens or a trial lens, nor is it meant to help the patient relax during the procedure. The focus of the pin's function is purely on providing a reference point for accurate alignment during keratometry.