The importance of forbidden line radiation is considered, taking into account lines in the visible and the infrared range. The result of a theoretical study conducted by Petrosian (1970) is shown in a graph which indicates the intensities of the IR lines predicted for M42. The predicted values were found later to be one order of magnitude too high. A description is given of a spectrometer which was designed for extraterrestrial observations. The spectrometer was used together with a 91-cm telescope which is installed in a NASA Convair 141 aircraft. Several observational flights were conducted in November and December of 1975. The results of the studies are discussed, taking into account the detection of the forbidden line at 18.7 micrometers in M42.