Which instrument is used to measure the extension of the anterior surface of the cornea beyond the lateral orbital rim?

The exophthalmometer is specifically designed for measuring the degree of proptosis, which is the forward displacement of the eyeball. This instrument quantifies how much the anterior surface of the cornea extends beyond the lateral orbital rim. By providing accurate measurements, the exophthalmometer helps in diagnosing conditions such as Graves' disease, tumors, or other conditions that can cause the eyeball to protrude.

In contrast, an ophthalmoscope is used for examining the interior structures of the eye, such as the retina, and does not measure external protrusion of the eye. A slit lamp is mainly utilized for a detailed examination of the anterior segment of the eye, including the cornea, but it also does not provide measurements of protrusion. A tonometry device is used to measure intraocular pressure, which is not related to the assessment of the eye's position relative to the orbit. Hence, while all other instruments serve important functions in ophthalmology, only the exophthalmometer is designed for measuring the extension of the cornea beyond the orbital rim.