 Dissectors were used only briefly in television systems before being replaced by the much more sensitive iconoscope during the 1930s.An image dissector focuses a visual image onto a layer of photosensitive material, such as caesium oxide, which emits negatively charged "photoelectrons" proportional to the intensity of the light striking the material.Electrostatic deflecting plates or magnetic fields then periodically manipulate the resulting electron image horizontally and vertically before an electron multiplier, or a small aperture leading to a positively charged detector, or anode, in the case of the earliest dissector tubes.