JF Ptak Science Books Quick Post
In an earlier issue of Nature, back on January 15, 1880, there appeared a wonderful article by Francis Galton that turned out to be one of the earliest treatments of synesthesia. “Visualized Numerals” 1 concerned (in one aspect) the mental visualization of the act of doing mathematics. That is to say, what images people formed in their brains as they performed mathematical functions. It was also an illustrated article making it perhaps the first attempt at visualizing a thought process.
Browsing another issue of Nature for May 6, 1890, there appeared another unusual and related piece, this by George N. Newton, "Visualized Images Produced by Music" (reproduced below).
This was probably nothing new to a lot of people, least of whom included tone poem composers and others who heard music and experienced color sensations--but this too, like the Galton, was a very early publication on the subject. Or at least in a reachable, non-trivial, not-obscure publication--there were instances of articles written earlier in the 19th century, though they did not find an audience like that of Nature. Folks have been thinking about color hearing for thousands of years, though not necessarily writing much about it or (later) publishing on it--to these people this article was preaching to the choir, in color.
1. See "A New Class of Scientific Image? Synesthesia, Francis Galton and Picturing Thought", here.
Also for a good site with many links, see the Center for Visual Music, here.