Which instrument would most likely be used to measure intraocular pressure?

The tonometer is the instrument specifically designed to measure intraocular pressure (IOP), which is critical in diagnosing and managing conditions like glaucoma. It assesses the pressure within the eye by measuring the resistance of the cornea when a small force is applied. There are various types of tonometers, including applanation and non-contact tonometers.

In contrast, the retinoscope is used primarily to determine refractive errors by evaluating how light is reflected off the retina. The ophthalmoscope serves to visualize the inside of the eye, particularly the retina and optic nerve, facilitating insights into overall eye health but not measuring pressure. Lastly, the slit lamp is a microscope used for examining the anterior segment of the eye, including the cornea and lens, but it does not measure intraocular pressure. Thus, the tonometer is the clear choice for specifically gauging pressure within the eye.