Far away from Dublin and relying on Thom's, Joyce appears to have gotten one detail of the cityscape very slightly wrong in Calypso. The narrative says of Bloom, "He crossed to the bright side, avoiding the loose cellarflap of number seventyfive." His house sits across from number 77.
Bloom's house sits on the northeast side of Eccles Street, facing away from the morning sun, so he crosses to the warm and cheerful side of the street. There he turns left, to get to Dorset Street. In James Joyce's Dublin, Ian Gunn and Clive Hart write, "As on one or two other occasions, Joyce overreached himself in using Thom's to supply the meticulous factual background which would give the reader such an uncanny sense of close observation and of remarkable powers of recall. The houses opposite no. 7 Eccles Street are nos 76 and 77. To reach no. 75 Bloom would have had to veer slightly to his right, away from his intended path" (32-33).
"It is of course possible," Gunn and Hart concede, "that what appears to be a mistake is a halfhidden indication that Bloom begins by walking in the other direction and then changes his mind. If so, that provides an interesting adumbration of the end of Lestrygonians, when Bloom, about to cross sunny Kildare Street on his way to the Library, veers to the right to avoid Boylan. It is perhaps more probable, however, that this is an error arising from Joyce's having used the street list in Thom's to count seven houses back on the 'bright' side, beginning with the last, no. 81, and thus arriving at 75. In physical fact the corner of Eccles Street and Dorset Street, on that side of the road, is occupied not by the last house in Eccles Street but by 72-73 Upper Dorset Street (Larry O'Rourke's pub), while on Bloom's side no. 1 Eccles Street is on the corner. This matter, of minimal importance in itself, is an example of how Joyce's working methods could occasionally lead him astray" (33).