In the grammar section of Pharr and Wright’s Homeric Greek section 601, the following is stated:
The loss of one or more consonants in a word usually occasions the lengthening of the preceding vowel. This is called compensative lengthening. When it takes place, α, ι, υ = ᾱ, ῑ, ῡ; ε = ει; ο = ου.
I know that in later Greek, the pronunciation of ᾳ, ῃ and ῳ was as though there was no iota subscript (§ 506), but ‘in Homeric times the iota was probably sounded to some extent’ (ibīdem). Similarly, the dipthongs are all listed (§ 504) as having both vowels pronounced.
For these reasons, I would assume that compensative lengthening should have yielded the following:
- α → ᾱ
- ε → *η
- ι → ῑ
- ο → *ω
- υ → ῡ
With my very sparse knowledge of Greek (I am still very much a beginner student), I can think of only three reasons for having ε → ει and ο → ου:
- Spelling conventions, similar to Old Latin EI for Ī, e.g. DEIVIDVNDA (Ward, Ralph L.: _Evidence for the Pronunciation of Latin (Continued)’, The Classical World, Vol. 55, No. 9 (Jun., 1962), pp. 273–275). ɔ: Though spelt ει and ου, they were in actuality pronounced η /eː/ and ω /ɔː/ rather than /ɛː/ and /uː/.
- These vowels actually changed to /ei̯/ and /ou̯/ when compensatively lengthened.
- These vowels actually changed to /ɛː/ and /uː/ when compensatively lengthened.
Why did ε and ο get compensatively lengthened to ει and ου and how were they pronounced? If we here have an example a change in vowel quality, what prompted this?