John Constable’s close friend John Fisher died on the afternoon of August 25, 1832 – but the original painting was first exhibited in 1831.

Professor Thornes’ findings explain how a re-assessment of the solar geometry of the painting, and Constable’s understanding of contemporary rainbow theory, suggest that the rainbow was added in at a later date as a homage to Mr Fisher.

The end of the rainbow can even be seen to rest on Mr Fisher’s house, where Constable stayed during his visits to Salisbury.

Mr Thornes said: “Constable was a great believer that painting is a science, something that should be pursued with the aim of understanding the laws of nature.

“This approach is clearly applied to the clouds and weather in his works, but it was not the case with all of his depictions of rainbows.

“Unlike clouds, rainbows are seen much less frequently in his work and were therefore often more mysterious in their symbolic function.”