Figure
9 is constructed to illustrate the principle. The left and middle images
appear identical to the eye, but they are not. In the middle image, the Palaeontologia
Electronica logo has been superimposed on the original picture and given an
opacity value of 1% (i.e., it is so transparent as to be practically invisible).
When the middle image is subtracted from the left one and the levels
are adjusted, the logotype appears again (right image), as an expression of the
areas in which the two images differ ever so slightly. Note that the third image
can only be recovered when the first two, imperceptibly different images are
combined to interfere with each other; thus the third image can be regarded as
being embedded in both, not just one, of the first two.
This case is constructed, but the principle can often be profitably applied to palaeontological imaging. If different parts of the object reflect light differently, in frequency or polarization, two or more recordings can be made to interfere with one another so as to bring out the regions in which they differ. In this way, a picture taken in plain light can be pitched against one taken with crossed nicols. Another possibility is to magnify colour differences in an object by using images taken in different parts of the light spectrum.
