tenex, post 77,
A polished silver layer exposed to air oxidizes very quickly, it turns black (basic high school chemistry). When kept under a nitrogen atmosphere in a roof prism binocular the chances are very small that this occurs. It becomes almost zero, when the silver layer is protected by a spectrally neutral transparent layer. Nevertheless it sometimes occurs as I found out with older binoculars which needed to be repaired for that reason. But that is no reason for concern, since it very rarely occurs.
Gijs van Ginkel
The tarnishing though is silver sulfide caused by minute quantities of hydrogen sulfide in the environment.
Irrespective of whether there are itermediate dielectric coatings between the prism surface and the silver reflective layer, the protection layer does not need to be transparent. It could be completely opaque.
John