Imagine looking down a spiraling coil. From end to end, it appears as a tube, spiraling away to a single point.
X-ray Photo 51: Spots form an x shape which indicates a helix if you're looking down the center of the DNA molecule.
That's what I see a hint of when I look at Photo 51, like I'm spying down a tube of DNA (see the vertical line down Figure B below).
Here's another view.
