We are all familiar with the "Then & Now" approach to comparing contemporary and historic landscapes and urban settings. First we see a photograph of a building (or street or whatever) as it looks today. Then we compare it to another photo of the same site 50, 75, 100 years ago. It is a compelling way to emphasize change (or permanence, I suppose, though it is seldom used that way).
But I had never seen a "Then & Now" like the Museum of London has developed. Called Streetmuseum, it is an app that blends the then and the now in a single, hybrid image. You can see how it works at the BBC site. Very cool.