Black to move
Morphy played 22...Bg2+, which wins. Beim writes,
As Zukertort later pointed out, another forced win was 22...Rg2! 23.Qd3 Rxf2+ 24.Kg1 Rg2+ 25.Kh1 Rg1#.Beim's bibliography at the end of the book lists nothing by Johannes Hermann Zukertort. Nor is there any thing in the text that would lead a curious reader to a secondary source mentioning Zukertort's analysis, unless perhaps reading everything listed there. Publishers understand that footnotes and endnotes do not sell books, and thus they mak minimal demands upon authors to document their assertions.
Beim, Paul Morphy, 46.
In my history blog, Patriots and Peoples, I have written extensively about documentation. One text that I discuss with some frequency there has abundant references, but examination of these texts often reveals that they present information and arguments almost opposite what is asserted in the referring text. Historians must document their work--it is an expected professional standard, and they must do so with honesty and accuracy. Why may chess historians adhere to a lower standard? They should not do so. Good chess history must meet the standards of good history.
Despite Beim's failure to document, Google Books quickly offered help. A search for Zukertort as author turned up volume 8 of The Chess-Monthly (September 1886 - August 1887). The February 1887 issue contains an article by Zukertort, "The Morphy-Paulsen End-Game" (171-173). Zukertort mentions several discussions of Paulsen -- Morphy in the chess clubs of London, Vienna, and New York. Writing about one of the discussions at the St. George's Chess Club of London, he states:
On one of these occasions (about the end of 1880) I pointed out that Morphy, in his masterly tournament game with L. Paulsen, overlooked both a mate in six and then in four moves, with no desire to detract from the general play of the great master, but just to illustrate the carelessness of all his commentators of the past. (172)He also claims that he showed it to Paulsen two years later, and that led to discussion among masters in Vienna.