Optical cement. Traditionally, Canadian Balsam would have been used, but there are newer adhesives that are apparently easier. I bet UV cured is a lot more practical in a production setting. See this article about using balsam to repair a doublet:

http://forum.mflenses.com/re-cementing-doublet-elements-with...

Most likely one or another optical cement, for joining lenses into a lens group, for example: https://www.cloudynights.com/topic/661086-using-norland-uv-c...