Henshall seems to have a quite specific idea about how many phonetic elements end up in characters, which diverges from explanations I find online, but I'm not sure I'm reading him correctly. Essentially, I'm wondering if he takes the view that for many characters, the sound was not strictly indicated by the 'phonetic component', but instead that a semantic link (a kind of ideograph) was created using a phonetic substitute.
An example from the list would be 映, where 日 is semantic and the radical, while 央 is said to be "acting phonetically to express bright". He says it is probably also lending its meaning, i.e. "centre" ('centre of sun' being especially 'bright'). This implies a distinction between "acting phonetically to express" a meaning, and "lending" a meaning.
In the introduction his explanation also implies this distinction. He says that the character 注 is made of 水 as the radical and 主, where 主 is a "phonetic element which expresses the sound of a word meaning continuous (specifically 続)". He then glosses the character elements as signifying 'continuous' combined with 'water' (writing: "thus continuous (flow of) water").
To me this suggests that 主 is not to be understood as giving the sound for 注, but instead as providing a semantic element, "continuous", normally written as 続, using a simpler character which at some point had the same sound as 続. This is not because it couldn't be the case that 主 is both phonetic and semantic, but because of Henshall's actual phrasing of this explanation, which seems carefully set up to suggest that 主 is being used to mean continuous, by sounding like continuous, rather than give the sound for 注.
I'm not at all read up on scholarship around this of course, just curious to find this quite distinctive understanding of phonetic characters in this relatively standard book for helping learners understand Kanji origins. Does anyone know if I'm reading him right, and if so - is his an orthodox view of these phonetic elements?