Illustration by Henry Holiday to The Hunting of the Snark (1876, chapter The Vanishing) and The Image Breakers (1566-1568) by Marcus Gheeraerts the Elder.

How does blurring help to compare the images? The Priest in the Mouth detail is displayed using two high resolution images (middle) which then again have been low pass filtered (bottom). That filtering helps to focus on larger structures.

This was the first allusion by Henry Holiday to another work of art which I discovered in December 2008.