Nystagmus is a series of more or less rapid vibrations that take place when the eye is in or near the primary position. In direction, the oscillations are generally horizontal, but they may be vertical, rotary or mixed. The frequency varies from 60 to 200 separate oscillations per minute, and the extent of the excursions from 1 to 10 mm. True nystagmus is almost always binocular, and the movements are conjugate, i. e., equal, simultaneous and parallel in the two eyes. This means that the two eyes act alike; it does not imply that the two components of the oscillations are equal, for, as is well known, there are two distinct types of oscillation, the springing or resilient and the pendulum.
The resilient type of nystagmus has a slow component in one direction which is immediately followed by a quick return. The slow component is vestibular and is characteristic
KNIGHTON WS. HEREDITARY NYSTAGMUSREPORT OF A CASE. Arch Ophthalmol. 1929;2(4):437–442. doi:10.1001/archopht.1929.00810020455006