... Ultimately, he derived a formula relating the temperature of the gas in nebulae to the intensity ratio of forbidden lines observed in their spectra. The method of determining the temperatures of nebulae from the intensity ratios of forbidden lines in their spectra was proposed on the basis of these studies. ... 13) This ratio has a very weak temperature dependence. ... The electron temperatures of a large number of nebulae have been determined from the intensity ratios of forbidden lines. ...