Alexander's law
Peripheral nystagmus grows toward the fast phase but never reverses.
What the animation does not show. The sign is the intensity GRADIENT across gaze, and the animation does not show it. It holds one fixed amplitude in one gaze position, so what you see here is the fixed direction, not the strong-toward-fast, weak-away grading that the law is actually about. Judge the gradient by looking in each direction at the bedside.
A unidirectional vestibular nystagmus is strongest looking toward the fast phase and weakest looking away, and it keeps the same direction across gaze. That intensity gradient is peripheral. A fast phase that switches direction with gaze is gaze-evoked and central instead.
Alexander's law describes how a peripheral vestibular nystagmus behaves across gaze: strongest looking toward the fast phase, weaker in centre, weakest looking away — but never reversing. That fixed direction with a graded intensity is the peripheral signature.
It is the direct counterpart to gaze-evoked nystagmus. If the fast phase switches direction with gaze, the sign is direction-changing and central, not Alexander's law. Reading intensity-versus-direction across gaze is one of the highest-yield distinctions in the dizzy patient (Kattah 2009; Leigh & Zee).
Source: Leigh & Zee; HINTS exam (Kattah 2009).