Which of the following instruments can be used to measure both corneal curvature and the base curve of a rigid contact lens?

The keratometer is a specialized instrument designed to measure the curvature of the cornea, providing valuable information crucial for contact lens fitting. It uses reflected light to determine the shape and diameter of the corneal surface, which is essential for ensuring that a contact lens will fit properly on the eye.

Moreover, the keratometer can also be used to measure the base curve of a rigid contact lens. By placing the lens on the device, the same principles of curvature measurement apply, allowing practitioners to assess the fit of the lens against the cornea. This dual capability makes the keratometer a vital tool in both assessing corneal health and ensuring appropriate contact lens fitting.

While the lensometer is effective for determining the prescription of finished lenses, it doesn't measure curvature. The biomicroscope, also known as a slit lamp, aids in examining the front structures of the eye but does not measure curvature directly. The radiuscope measures the base curve of rigid contact lenses but does not measure corneal curvature. Thus, the keratometer stands out as the instrument capable of both functions, making it the correct choice.