What is the diameter of the plastic prism comprised of a flat surface within the applanation tonometer?

The diameter of the plastic prism within the applanation tonometer is indeed 3 mm. This is significant because the applanation tonometer is designed to measure intraocular pressure by flattening a specific area of the cornea, and the size of the prism has been precisely chosen to ensure accurate measurements while minimizing the impact on the corneal surface. The 3 mm diameter ensures that enough area of the cornea is interacted with to obtain a reliable and consistent reading of the ocular pressure without causing excessive stress or damage to the eye. This design is critical in ocular diagnostics, helping in the early detection of conditions such as glaucoma.