An early mis-step was the (incorrect) measurement of the energy value as 3.5±1.0 eV in 1994. This appeared fortunate for the topic because light of that frequency propagates both in air and in a number of crystals, while the correct frequency cannot propagate through air, requiring the experimental apparatus to be placed under a vacuum. In particular, this energy was comfortably below the 6.3 eV ionization energy of thorium (implying that decay by internal conversion was impossible even in neutral thorium, see § Ionization) and the 6.2 eV limit of ultraviolet transmission through molecular oxygen (air). Thus, direct detection experiments were attempted which had no hope of detecting the ultraviolet light at the true, higher, energy.