Frankel's first important volume appeared at Leipsic in 1841, under the title of _Die Vorstudien zu der Septuaginta_ (“Studies in the Septuagint”). Several other writers, including the renowned Jewish critic Azarya de Rossi (1514–78), had already devoted their earnest attention thereto, but none of them attained the same measure of success as the author of the _Vorstudien_. Frankel throws much light on the genesis, the composition, and the tendency of the Septuagint, and explains the reason why its appearance was hailed with delight by the Alexandrian Jews, but condemned by that section of the Jews which then resided in Palestine. Equally instructive and interesting are the remarks he makes on the influence which it exercised on several of the subsequent ancient translations of the Bible, and on the development of the so-called _Halacha_. But, he says, it has done much more than that. It has infused for the first time into the thoughts and language of the civilized nations of antiquity the lofty teachings of Judaism, and has directly and indirectly enriched the world of letters with many suggestive ethical and moral lessons. By the publication of this particular book Frankel accomplished two important things: in the first place he showed the world that the Jews, as the authors of the Septuagint, have contributed a great deal to universal literature; and then he also made his own people acquainted with some of the precious literary treasures which they possessed, but which they had hitherto ignored. His second important work, published at Leipsic in 1859, is entitled _Darkè Hammishnah_, but it has also the following additional heading in Latin _Hodogetica in Mishnam librosque cum ea conjunctos, Tosefta, Mechilta, Sifra, Sifri_. It is written in an easy Hebrew style, which the author no doubt rightly thought would be thoroughly understood by all students of Rabbinical literature, as were not well versed in any other language. This volume contains, as its two titles already briefly indicate, much information on the ancient Rabbinical lore and tradition generally, but chiefly in respect of the Mishna. The latter is fully described there with regard to its many authors, its varied characteristics, its idiom, and its literary and scientific value. It seems scarcely credible, but it is a fact that this important and highly instructive work was at the time of its appearance vehemently assailed by some ultra-orthodox German and Austrian Rabbis, who publicly declared it to be a dangerous book, and one calculated to undermine the very foundations of Jewish law and tradition. On the other hand it is gratifying to note that Rapoport at once recognized its great merits, and did his best to defend it.