昨日の記事「#5535. 接頭辞 en- と in- の揺れを Chancery English でみる」 ([2024-06-22-1]) で取り上げた話題について，もっと調べてみたくなり OED の en- PREFIX¹ を引いた．語源欄の Note 3 に，この揺れについて詳しい歴史的経緯が記されている．勉強になったので，その箇所をすべて引用しておく．

3. From 14th cent. onwards the prefix in- (im-) has been frequently substituted for en- (em-); and, conversely, en- (em-) has been substituted for the prefix in- (im-) of words of Latin or Italian origin, and for the native English in- prefix¹. Nearly every word, of long standing in the language, which is formed with en- has at some period been written also with in-. Hence it is often impossible to determine whether in a particular word of English formation the prefix en- or in- is due to the analogy of words of French, Latin, or purely English origin; in many instances it must have been applied merely as a recognized English formative, without reference to the analogy of any individual word. In 17th cent. the form in- (im-) was generally preferred; the now prevailing tendency is to use en- (em-) in English formations, and where the prefix represents French en-; and in modern reprints of 17th cent. books, and in dictionaries, the in- (im-) of the original texts is often replaced by en- (em-). In some words, however, as em-, imbed, en-, inclose, the form with in- still occurs, but in most cases less frequently than the en- forms; in a few instances in- has entirely superseded en-, even where the latter is etymologically more correct, as in imbrue, impair, inquest. In a few words (e.g. ensure v., insure v.) the alternative forms have (in very modern times) been appropriated to express different senses. As a general rule the en- and in- forms are in this Dict. treated as belonging to one and the same word. A word still surviving in use is treated in the alphabetical place of its now more frequent form. In the case of obsolete words, where there is no decided preponderance in usage, the choice of the typical form has been determined by etymological considerations: thus the adapted words from French or Spanish with en-, and new formations apparently on the analogy of these, are by preference placed under E; while words apparently formed on Latin analogies, or probably originating as compounds of the English preposition in n.², will appear under I.

The substitution of in- for en- has in part been due to notions of etymological fitness, the Romanic en- having been regarded as a corrupt and improper form of the Latin in-, while the English formations in en- were either referred to Latin analogies or treated as compounds of the native preposition. The phenomenon seems, however, to be partly of phonetic origin. Tendency to reduce and slightly raise the vowel in this prefix results in homophonic pronunciations of word pairs such as embed and imbed, enclose and inclose. Occurrence of spellings such as inbassed for embassade in the fourteenth century may suggest this phenomenon has existed from an early period.

　揺れの要因についての議論が詳しいが，数々の要因が作用しているようで現実はきわめて複雑だ．語源がフランス語かラテン語かという要因はもちろん重要だ．しかし，個々の単語においては各々の語形が互いに乗り入れしており，緩い傾向があるような，ないようなという状況だ．さらに，英語内部での語形成の場合には，フランス語やラテン語は，類推のモデルとしてあくまで間接的に作用しているにすぎず，結局いずれの接頭辞かを決める主要因が何なのかが，しばしば分からない．これは似非的なヴァージョンを含む語源的綴字 (etymological_respelling) をめぐる議論にも近似してくる．
　最初の段落の終わりにかけては，この揺れが辞書編纂者の視点からも悩ましい問題であることが示されている．OED では接頭辞 en- と in- を原則として同一物として扱うが，では，どちらの接頭辞で主見出しを立てるべきか，という実際的な悩みがここに吐露されているものと読める．
　英語綴字の歴史的深み（闇？）をもう1つ知ってしまった．

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