1. The current English translation of ‘anschaulich’ is ‘visualizable’, but except in quotations I shall use the word ‘intuitive’ in order to keep a more explicit reference to the Kantian tradition. From the point of view of that tradition, the word ‘intuitive’ makes sense in the context of all the original papers on quantum theory.
2. See e.g. APHK, 38, or the first section of Bohr (1939), which is one of Bohr’s most elaborate papers. On his use of the word ‘symbol’, see also Folse (1985), 246–9; Honner (1987), 153–160; and Chevalley (1991a), 559 sq.
3. See also ATDN, 12: “the symbolical garb of the methods in question closely corresponds to the fundamentally unvisualizable character of the problems concerned”.
4. Such importance was clearly assessed at the time; see e.g. Heisenberg’s 1929 statement that BKS “contributed more than any other paper from that period to the clarification of the situation in the quantum theory” (Works 5, ix). On the BKS theory, see Stolzenburg’s introduction to Works 5; for a connection with the concepts of Bild and Anschauung, see Chevalley (1991a), 458 sq.
5. Ever since 1913, Bohr had also been aware that in order to speak about the energy and momentum of light quanta, one had to use the concepts of frequency and wave length, as the very formula E = hv illustrated.