Note the all-important long dash separating those two words. The word 'medium' belongs to the previous phrase - ' a slightly more unusual, but common, medium' - while 'banknotes' stands alone.
In this sentence, the word 'medium' is a noun. It refers to the form that something takes or what it is made of. We use the word a lot in the arts. For example, clay is a medium that a potter would work with.
The usual medium for memorializing famous people has traditionally been oil paints or stone/marble/bronze i.e. portraits and statues. Nowadays we often put their pictures on banknotes. Banknotes are common everyday objects, but they are an unusual medium for memorializing people.
The dash is telling us that the banknotes ARE the unusual but common means of immortalizing the scientists.
I hope that makes sense.