I'm trying to understand this image of a particle tracing experiment (which can be found all over the net if you google for "bubble chamber"):
There are two things that I can't figure out:
The background has some kind of pattern made up of irregularly sized, semi-rectangular patches. Where does that come from?
There is a vertical line of 4 circles in the middle of the image, each marked with an X. These seem to be part of the setup, but what is their function? Also, the pattern in the background in these circles is not continuous w.r.t. to the outside pattern, although the traces are. Why?
In addition, if anyone has the original source or more information about that particular experiment then that would be very welcome, too.
EDIT: It seems that the image stems from a 1980 Fermilab experiment.