For Herder's correspondence, see Aus Herders Nachlass (3 vols., 1856-1857), Herders Reise nach Italien (1859), Von and an Herder: Ungedruckte Briefe (3 vols., 1861-1862) - all three works edited by H.
(1880-1882); the Litterarischer Nachlass, a posthumous collection, was published at Vienna (1887).
Darer (Nuremberg, 1827); again, edited by Thausing, in the Quellenschriften far Kunstgeschichte und Kunsttechnik (Vienna, 1872), but most completely in Lange and Fuhse's Diirers schriftlicher Nachlass (Halle, 1893); W.
Arthur Schopenhauers handschriftlicher Nachlass was published by Grisebach in 4 vols.
Running through these volumes in order, we have in the second the memoir, Summatio quarundam serierum singularium, the memoirs on the theory of biquadratic residues, in which the notion of complex numbers of the form a--bi was first introduced into the theory of numbers; and included in the Nachlass are some valuable tables.
The Nachlass contains further researches on this subject, and also researches (unfortunately very fragmentary) on the lemniscate-function, &c., showing that Gauss was, even before 1800, in possession of many of the discoveries which have made the names of N.
Leitzmann, Aus Lichtenbergs Nachlass (1899).