Claims I'm evaluating
I've read in multiple sources that Leibniz formulated separation of variables for ODEs in 1691. A couple example sources are below.
Does anyone know the original source for this claim?
Note 1: If it's a handwritten document (e.g. a letter), then I'd be delighted to see a digital copy of the original.
Note 2: If there are English (or Spanish) translations available, that would be helpful.
Note 3: If the original source is not available online, then any information about the specific equation (or equations) that Leibniz solved, or the context in which the equation arose, would be interesting.
Note 4: If anyone happens to know where I could find John (Johann) Bernoulli's Acta Eruditorum article of 1694, in which he apparently elaborates on the method of separation of variables, then that would be helpful as well.
What I've found so far
The closest source I've found is a letter from Leibniz to Huygens, written on the 29th of December, 1691. The letter is in French, and I read French only a little (just what I can pick up from knowing another romance language). Nonetheless, Leibniz clearly applies separation of variables in this letter to the equation dy = y/a dx.
In Kline (see link above), Leibniz's introduction of separation of variables is said to occur in a letter to Huygens, but I don't see any references to a specific letter. Since Leibniz and Huygens corresponded quite a bit in 1691, it's possible that another letter is being referenced.
Thank you for any assistance you can provide!