I can help out in this particular query, and throw - pardon the next line - some light upon it! This picture is of a gate housed in the vaults running below and adjacent to Blair Street, just up from the area known as the Cowgate. The gate itself - as is the entire network of vaults here - is beneath the ground, with no daylight getting into the set of rooms. So in this particular photo I can say with extreme certaincy that the light source here cannot be daylight. Actually, it's more to do with the flash on the camera. Basically, the camera flash has illuminated the reflective sign on...
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