Minolta XD-11 Scale-Illumination Window

[JeremyLangford]

I've read that the little box above the "i" in Minolta is a "Scale-Illumination Window". What does it do?

 

[bkrystad]

The XD-11 displays both the aperture and shutter speed in the prism in SP and M modes. The illumination window provides the light required to reflect the shutter speed into the prism. The aperture is visible through the small window directed down onto the aperture ring of the lens.