Holtzmann Adolf Holtzmann 
‚Uber den Umlaut: Zwei Abhandlungen‘ (Carlsruhe, 1843) and ‚Uber den Ablaut‘ (Carlsruhe, 1844) [PDF ebook] 

Uber den Umlaut (1843) and Uber den Ablaut (1844) grew out of a review of Grimm’s Deutsche Grammatik by Holtzmann, in which he also made an excursus into Bopp’s theory of vowel gradation in Sanskrit. Holtzmann was the first to observe the correlation of guṇa and accent. At the same time he noted that loss (or absence) of the accent could mean loss or shortening of a vowel. Observations which, be it in a different form, eventually found their way into a unified theory of Indo-European vowel gradation.The two German texts are presented here in fac simile format, together with an introductory article.