What is the prism tolerance for a mounted pair of eyeglasses in the horizontal meridian according to ANSI standards?

The standard for prism tolerance in the horizontal meridian for mounted eyeglasses according to ANSI (American National Standards Institute) standards is indeed two-thirds of a prism diopter. This specification is important because it ensures that any small misalignment or deviation in the eyewear does not significantly impact the wearer's vision or comfort.

Having this standard helps maintain optical quality and ensures that lenses are manufactured with precision. If the mounted glasses deviate beyond the specified two-thirds of a prism diopter, it could lead to visual discomfort or visual distortion for the wearer. Thus, the two-thirds prism tolerance serves as a critical measure for the quality assurance of eyeglasses, ensuring that they meet the necessary requirements for proper vision correction and alignment.