I've just been examining my collection of 35 mm colour positive slides and I have noticed something unusual. The edge of the colour positive slide, where the image meets the inner rim of the border it is housed in, is a lighter shade compared to the rest of the picture. In some slides it may be the bottom edge and the right side of the image. In other slides it may be all four edges.
Does anyone know why this is?
Additional information: The lighter borders are only visible when viewing the slides over a dark area. In normal light the slides look perfectly normal.