Forcing free fusion of stereograms

n order to read articles that either are on stereopsis, or use stereopsis as a tool, it is virtually inescapable to learn to fuse stereograms without the help of a stereoscope or special glasses (free fusion). For a few, this is easy. For many, it is frustratingly difficult, leading some to turn their backs on stereopsis altogether. Here, we show how presenting a stereogram on a transparency-rather than on paper-with a fixation cross behind it (for uncrossed fusion), or presenting a fixation cross on a transparency with a paper stereogram behind it (for crossed fusion), can force free fusion towards greater success.