  We present mid-IR images of a 15'x15' field in the south-west part of the
Andromeda galaxy M 31 obtained with the ISOCAM camera on board ISO. We also
present a 20" resolution far-UV image of a larger field at 200 nm obtained with
the balloon-borne telescope FOCA 1000. These images are compared with HI,
CO(1-0) and Halpha maps. The mid-IR emission as seen through filters at 7 and
15 mic is extremely well correlated with the distribution of neutral gas as
shown by the HI and CO(1-0) maps, while the correlation is poorer with the
distribution of the ionized gas seen through its Halpha emission. There is some
correlation with the UV radiation, but it appears that the contribution of UV
photons to the excitation of the carriers of the mid-IR emission is not
dominant in most of M 31. The spectro-imaging observations of Cesarsky et al.
(1998) show that the mid-IR spectra of several regions of M 31 are dominated by
a strong emission band at 11.3 mic while emission in the other classical
Infrared Bands at 6.2, 7.7 and 8.6 mic is faint or absent. More precisely, we
find that the mid-IR spectral variations are not clearly related to the UV
radiation field. The present observations have important consequences on our
understanding of excitation of the interstellar mid-IR emission. In particular,
we conclude that like for M 31, excitation in the Galactic cirruses may not be
dominated by UV photons but rather by another mechanism which remains to be
identified (visible photons ?). The UV excitation appears to become important
when the UV radiation density is of the order of twice that near the Sun.
