When Dreyfus was granted a retrial in 1904, Appell served as an expert on the commission that, by July 1906, had cleared Dreyfus and reversed all previous convictions.
Kapag Dreyfus ay ipinagkaloob sa isang panibagong paglilitis sa 1904, Appell nagsilbi bilang isang eksperto sa komisyon na, sa pamamagitan ng Hulyo 1906, nagkaroon nalilimas Dreyfus at baligtarin ang lahat ng nakaraang convictions.