Observations of faint H II regions in our Galaxy.

We present photoelectric observations of faint H II regions of emission measure lower than 500 cm^-6 pc at Hα. From these observations it is found that the limiting sufface brightness in Hα for detection of an H II region in the Red Palomar Sky Survey is smaller than 2.75 x 10^-6 erg cm^-2 s^-1. By comparing the observations by Reynolds et al. (1974) with the Red PSS we find that the limiting value lies in the 2.5 x 10^-6 to 5.5 x 10^-6 erg cm^-2 s^-1 sr^-1 range which for an unreddened nebula at Te = 6000 K corresponds to an emission measure between 17 and 40 cm pc. The mechanism of ionization and the chemical of two of the observed regions, beloging to the Barnard Loop Nebula and located about 4.5 away from the Orion Nebula, are studied. It is found that the relative abundances of N, 0 and H are not the same in both nebulae. Some arguments are given in favor of a normal nitrogen abundance for the Barnard Loop. By assuming that in this object the nitrogen to hydrogen abundance ratio is the same as for the Orion Nebula we derive an electron temperature in the 6000 - 7000 K range as well as an oxygen underabundance of a factor of 2 to 4.