Quote Originally Posted by Bernard_L View Post
Balsam is specifically used because it has an index of refraction close to that of glass. Not so with air.
Close to classic crowns and flints (1.5-1.6 or so). Not so much close to more modern glass types with indices of 1.7 or greater. For photographic objectives it's not as important. Very high performance designs have to account for the difference or they can run into trouble.