measured the specific charge of the alpha particles
by deflecting them in electric and magnetic fields,
the same method that J.J. Thomson had used to discover
had found the deflection of the alpha particles by measuring
their position on a photographic film. While making
the measurements, he observed an unusual effect; if
the alpha particles passed through a thin sheet of mica,
the images on the film were blurred.
images were sharp if the mica was not present. Something
about the mica sheet was causing the alpha particles
to scatter at seemingly random small angles, resulting
in the blurred images.