Which tool cannot be used to verify the overall gas permeable lens diameter?

To verify the overall diameter of a gas permeable (GP) lens, a radius scope is not suitable because it is specifically designed to measure the radius of curvature of the lens surfaces rather than its diameter. The radius scope primarily provides information on the curvature and does not address the linear measurement needed for diameter verification.

In contrast, a slot gauge is effective for determining lens diameter since it can accommodate various lens sizes and allows for precise linear measurements. A shadow graph also serves the purpose well by visually showing the lens dimensions in relation to a calibrated scale. Likewise, a measuring magnifier can be used to closely examine the edges of the lens and provide an accurate diameter measurement. Each of these tools has its specific application for measuring different attributes of contact lenses, but the radius scope is not appropriate for assessing overall diameter.