Coxsoft Art has pointed out that the National Gallery in London scanned the under-layers of Leonardo’s “The Virgin of the Rocks” painting. They used a technique called “Infrared Reflectography” which is similar to x-radiography. There’s more on infrared reflectograms at the Art Institute of Chicago for those that are interested.
They found that Leonardo had planned to have the virgin mother kneeling before baby Jesus. He abandoned the first version before he got around to drawing baby Jesus.
The National Gallery has much more about The Virgin of the Rocks here. For a close-up view of the infrared reflectogram of the painting, where the kneeling figure can be seen, see here (her clasped hands praying can be seen in the head of the virgin).
Art conservator Rachel Billinge said “It was an extraordinary moment when we pointed the camera on the Madonna’s face – just to get the settings right because the paint was thin there – and instantly we saw a hand which had no place there. We all had to go away and sit quietly for a bit, just to get our thoughts in order.” National Gallery News
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