A developing apparatus including a developer container which contains a first developer having a first charging polarity and a second developer having a second charging polarity opposite to the first charging polarity, a developer bearing member which bears the first and the second developers, a layer...http://www.google.de/patents/US7068967?utm_source=gb-gplus-sharePatent US7068967 - Developing apparatus with separable multiple developer layers and image forming apparatus incorporating same
