This Public Domain Review essay places Watts Hughes inside the wider nineteenth-century project of turning sound into marks: Scott de Martinville's phonautograph scratching speech into soot, Bell's photophone modulating a sunbeam with a vibrating mirror, and what Steven Connor calls a "conversion mania" among inventors of the period.
Against that background the eidophone looks less like an oddity and more like one solution among many, and an unusually beautiful one. Mullender-Ross is good on the ambiguity of the results — whether the marks were writing, measurement, or picture — and on the fact that Watts Hughes was a working singer and philanthropist rather than an institutional scientist.
The essay was published in November 2019 and is generously illustrated, including impression figures in pigment on glass held by Cyfarthfa Castle Museum and Art Gallery. If you only read one piece about her, read this and then her own 1891 article.