  Assuming the standard axioms for quaternionic quantum theory and a spatially
localized scattering interaction, the $S$-matrix in quaternionic quantum theory
is complex valued, not quaternionic. Using the standard connections between the
$S$-matrix, the forward scattering amplitude for electromagnetic wave
scattering, and the index of refraction, we show that the index of refraction
is necessarily complex, not quaternionic. This implies that the recent optical
experiment of Procopio et al. based on the Peres proposal does not test for
hyper-complex or quaternionic quantum effects arising within the standard
Hilbert space framework. Such a test requires looking at near zone fields, not
radiation zone fields.
