History of italicising variables and mathematical formatting in general

Question

In the modern day, especially with the advent of $\mathrm\TeX$, it is common practice to italicise variables. This can be seen as far back as Hann, J. (1850). Examples on the Integral Calculus. What's the oldest example of this and when did it become standard?

Answer 1

In another answer of mine, we see an image of "Disquisitiones generales circa superficies curvas" from 1828 by C.F. Gauss.

Note that already here the mathematical letters are set in italic. (But the operator $\mathrm{d}$ for derivatives is in roman.) I expect this practice goes back much further than 1828.

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Another. Institutiones Calculus Differentialis, L. Euler, 1787

Here is Analyse des Infiniment Petits, l'Hospital, 1696

Even then, using italic. Note the $d$ for differentials is italic as well.

Answer 2

Rene Descartes's La Géométrie of 1637 uses the same typographic convention used today in France: in equations, lower case letters are set in italics but capital letters are set in upright text. This can be seen on the third page of the book. It can also be seen in the 1925 bilingual edition by D. E. Smith and M. L. Latham, with a facsimile of the original and translation on facing pages. (Reprinted in 1954 by Dover.)

The web page https://debart.pagesperso-orange.fr/geometrie/index.html#oeuvre gives the same facsimile pages, as best as I can tell.