"The Light of Other Days" is a classic
science fiction short story by
Bob Shaw.
It is about slow glass, glass which takes light an extended period of time to go through. After its maufacture, it is typically placed in scenic locations, affording people with spectacular views. The thing that makes the story a classic is that slow glass works both ways. A character spends his days outside his house, looking in at the images of his dead wife, who lives on, for a few more months, in the depths of the slow glass.
The title of the Baxter/Clarke novel is a deliberate homage to this story.