For more than seven years past I have had in progress systematic photometric measurements on the brightness of the night sky as seen through certain colour filters. These filters are designed to isolate separate portions of the spectrum in the red, green, and blue. The green one isolates nearly as possible the light of the green auroral line due to oxygen, but the isolation is very imperfect, and, in fact, the continuous background of the spectrum often contributes more to the transmitted light does the auroral line.