hiː ðˈɛn spˈɛnt tˈuː jˈɪɹz ˌɔn klˈæsᵻfˌaɪd ɹᵻsˈɜːtʃ bᵻfˌoːɹ ɹᵻtˈɜːnɪŋ tə mˈaʊnt wˈɪlsən wˌɛɹ hiː kəntˈɪnjuːd tə kəntɹˈɪbjuːt ʌntˈɪl nˈaɪntiːnhˈʌndɹɪd fˈɪfti sˈɪks.
He then spent two years on classified research before returning to Mount Wilson where he continued to contribute until 1956.