Hi,
In Ansel Adam's 1981 edition of The Negative, there is a rather cryptic acknowledgement to articles by John L. Davenport in U.S. Camera, as forming the basis for the Zone System. I was curious about these articles for some time and was able to track them down, with no small help from a librarian. For others who might be similarly curious, I have uploaded them to archive.org:

http://www.archive.org/details/ConstantQualityPrints

As far as I know, the copyright was not reviewed on these, and they should now be part of the public domain.

I was very impressed by Davenport's articles, and hope that they will be of interest to others here.

David