  We describe data reduction and analysis of fluctuations in the Cosmic Far-IR
Background (CFIB) in large maps observed with the Multiband Imaging Photometer
for Spitzer (MIPS) instrument 160 micron detectors. We analyzed the
extragalactic First Look Survey (FLS) and the Spitzer Wide-area Infrared
Extragalactic Survey (SWIRE) Lockman Hole observations, the latter being the
largest low-cirrus mapping observation available. In the Lockman Hole map, we
measured the power spectrum of the CFIB by fitting a power law to the IR cirrus
component, the dominant foreground contaminant, and subtracting this cirrus
signal. The CFIB power spectrum at mid-high k (k ~ 0.2 - 0.5 arc min^-1) is
consistent with previous measurements of a relatively flat component. At lower
k, however, the power spectrum is clearly not flat, decreasing from our lowest
frequencies (k ~ 0.03 arc min^-1) and flattening at mid-frequencies (k ~ 0.1 -
0.2 arc min^-1). This behavior is consistent with the gross characteristics of
predictions of a source clustering signature in CFIB power spectra, and this is
the first report of such a detection.
