I was using the term Duplet from the source. I didn't make it up. I would trust a former Kodak head of optics over a wiki-hack (stupidly looked on wiki and found they used doublet). But Kingslake specifies that Doublet used in this fashion is ERRONEOUS. Please, do not mar this with thread with this argument any further. You imply it is not a proper term because you have never heard of it. Guaranteed you have many terms and such that I have never heard of and that does not debunk them in the least. Now, on to the regular answers.