What instrument is commonly used to check for lateral and vertical phorias?

The correct answer for identifying the instrument used to check for lateral and vertical phorias is the Jackson cross cylinder. This device is specifically designed to assess the alignment of the eyes by determining the presence and magnitude of phorias. Phorias are eye alignment issues that do not manifest as strabismus but rather involve the eye's ability to maintain proper alignment under certain conditions.

The Jackson cross cylinder contains two cylindrical lenses oriented at right angles to each other. By placing this instrument in front of a patient's eyes, the examiner can assess how the eyes respond to various orientations of light in order to understand their alignment and the presence of any phorias. This examination is particularly useful in diagnosing issues that may not be apparent without such optical assessments.

Other instruments listed have different primary functions. For example, a Maddox rod is used primarily for assessing horizontal phorias but is less versatile than the Jackson cross cylinder in terms of detecting both lateral and vertical phorias. A Snellen chart is used to measure visual acuity, while a phoropter is mainly utilized for refractive testing and determining the patient's prescription for glasses, rather than specifically for examining phorias.