Small differences between two complex and closely-similar photographs may be located by viewing the uniformly illuminated pair with a stereoscope. Provided the observer possesses normal stereoscopic vision, any non-common elements will appear to stand above the general picture plane provided by the fused common background. It is emphasised that the images being examined are not a conventional stereo-pair, although the left- or right-eye member of such a pair may be so compared with its counterpart in a subsequent stereopair. This stereoscopic technique was found preferable to the alternative 'blink' method for most scanning microscope images, although it is advantageous to have both methods available.
Mills, A. A.
"'Spot the Difference': Finding Small Differences Between Closely Similar Images,"
Scanning Microscopy: Vol. 3
, Article 14.
Available at: https://digitalcommons.usu.edu/microscopy/vol3/iss1/14