ME ę̄ was a close e [eː], and ME ę̄ an open [ɛː]. They were kept strictly apart in ME and EModE. ME ę̄ must have begun to move towards [iː] in the 15th century. About 1500 [iː] is attested for certain. Even when ME ę̄ was tensed to [iː], ME ę̄ was still open [ɛː] for some time. The raising of ME ę̄ to [eː], according to Dobson 1968^2, took place at least towards the middle of the I7th century, while Horn 1954 assumes that the development began in the latter part of the 16th century. After that ME ę̄ was raised to [iː] and fell together with the reflex of ME ę̄.
Wyld 1936^3 suggests that so far as the [iː] pronunciation in 'ME ę̄ words' is concerned, StE had adopted a pronunciation developed in some dialect. Dobson states that ME ę̄ in StE itself became [iː] by a gradual phonetic change is false. The [iː] pronunciation in 'ME ę̄ words' does not go back to ME ę̄ at all; it goes back to a late ME ę̄ variant, which has become PresE [iː]. Kökeritz 1953 in of his opinion on that point. This variant completely fell together with the reflex of ME ę̄.
Strangely enough ME ę̄ has become PresE [ei] in four words— great, break, steak, yea. Although it is quite doubtful whether the [ei] pronunciation in all of these four words is to be explained in the same way, the falling together with ME ā under [ei] took place in the 18th century. ME ā, which has become PresE [ei], must have begun to be fronted to [aː] in the 15th century, and then through the intermediate stages [æː], [ɛː], [eː], it was finally diphthongized to [ei] about 1800.
This paper aimes to clarify the development of ME ę̄ in the Modern English period, centering on the raising to the stage [iː], and consider when and how ME ę̄ was finally diphthongized to [ei] in four words— great, break, steak, yea. Taking great for example, Dr. Johnson said Lord Chesterfield told him that great should be pronounced so as to rhyme with state, while Sir William Yonge sent him word that it should rhyme with seat. It was impossible for seat to rhyme with state, because the former must have been raised to [iː] when the latter was tensed to [eː]. Then a question arises as to how great was pronounced in the early 18th century. To find the solution of this problem, rhyme evidences are collected and examined in this paper. As a consequence of this survey, it is made clear that great was frequently pronounced [griːt] so as to rhyme with ME ę̄ words like seat, etc., and, at the same time, [greːt] so as to rhyme with ME ā words like state, etc.