I was reading Watson and Crick's article on DNA structure, and the diagram on the lower left of the first page had something called the fiber axis going through the DNA. This axis isn't in modern models of DNA, so what is it?
Short answer: The term fiber axis is not in reference to the DNA model, it comes from the experiments that Watson and Crick used to guess their model. The fiber axis is basically the dimension along the length of the DNA strand.
Watson and Crick created their model of DNA based on the somewhat esoteric experiment called X-ray fiber diffraction. For the experimental data (Rosalind Franklin was actually gathering the fiber diffraction data) DNA is pulled out of a cell lysate and washed with buffer. It looks like a clear, liquid string of snot (a technical term that is). Its stringy though because the long DNA molecules pull out and create viscous fluid where the DNA is pulled along the length of the fiber. Because the DNA is ordered, if you shoot a beam of X-rays through it, it creates a x-pattern on film that looks like this:
As you can see its got a nice X-shaped pattern. This is because the DNA in the fiber is mostly aligned along the fiber, in the up and down direction. Because this is so, the spacings between the different layers is due to the spacings between the DNA bases and the x-pattern comes from the fact that DNA forms a double helix.
Some misc details - you have to use X-rays because their wavelength is about the size of an chemical bond and this is the sort of model Watson and Crick and Franklin and Wilkinson were trying to find.
The fiber axis is along the y axis (top to bottom) because the trail of DNA snot hangs down - it will sag if you hold it an angle.
Francis Crick had actually predicted the x-pattern for helices. The structure actually required these two to come together to get the answer while the American genius Linus Pauling was working hard in California on the structure as well.
The reference for nearly all of this is Watson's somewhat self-centered but scientifically accurate book "The Double Helix". After some surprisingly difficult Googling, I can't found a picture of DNA snot fiber... picture below. You can make some yourself from the protocol listed here and some saliva, soap and other common household items.