A small field in the galactic plane is proposed for mapping with
 ISO. The selected line of sight is dominated by material located in
 the high concentration of molecular clouds and HII regions complexes
 known as the "molecular ring", viewed tangentially.  A field with
 minimal confusion in this densely populated region has been carefully
 selected, free of active star forming regions, and where high
 resolution spectral maps of molecules are available, that provide
 detailed geometric information on all individual clouds along the
 line of sight. This information will be used to spatially decompose
 the ISO maps in contributions from the different components and
 clouds along the line of sight, in an analogous way to earlier
 studies of the local interstellar medium based on IRAS images.  The
 goal of the proposed observations is to provide accurate photometric
 measurements of the infrared emission from representative clouds
 bathing in the strong radiation field of the molecular ring.

 ISO is the only facility capable of sufficiently accurate
 measurements of extended emission at sub-pc scale in the molecular
 ring, over the infrared spectrum. In order to ensure the success of
 this nevertheless non-trivial proposal, the expertise of the ISOCAM
 calibration team at Institut d'Astrophysique Spatiale will be fully
 exploited for the data processing.

 These observations, closely coupled with the "Spectral Survey of the
 Cold Interstellar Medium" proposed by J.L. Puget, should help making
 significant progress in interstellar physics.  Providing details on
 the infrared emission from a normal disk galaxy, they will also be
 helpful in the interpretation of infrared data from external
 galaxies.