dispersive power
Webster's Dictionary (GCIDE)
Dispersive \Dis*pers"ive\, a. Tending to disperse. [1913 Webster]
Dispersive power (Opt.), the relative effect of a material in separating the different rays of light by refraction, as when the substance is formed into a prism. -- Dis*pers"ive*ness, n. [1913 Webster]