Sabatier photography is a technique in which the negative of a photograph is completely or slightly changed in tone. In a nutshell, dark is light and light is dark.
This technique can be created by over exposing the negative of an image or, as with almost all aspects of photography, can be created within Photoshop.
Some great articles on Solarization :
Solarization and Inversion »
Solarization on Wikipedia »
Unblinking Eye – Controlling the Sabatier Effect »
Could someone please tell me the proper spelling: is it Solarised or Solarized?