IN his review of The Edinburgh History of the Scots Language (April 30), Michael Fry states: ''Hugh MacDiarmid composed sublime poetry in his own form of synthetic Scots''.

MacDiarmid's early use of the word ''synthetic'' has proved to be unwise. In more recent times, this word has become associated with what is ersatz and artificial. As a result, there is now a widespread misconception that, in some sense, the language of MacDiarmid's poetry is concocted and unnatural.

Lallans was a term popular among some of MacDiarmid's later emulators, to describe his extension of the use of Scots, and in recent years a foolish belief has gained ground that MacDiarmid invented a new artificial language with this name. This was a claim he repudiated in his autobiography.

Although MacDiarmid revived a number of archaic words in some poems, in general the language in which he wrote was very natural; as in ''Crowdieknowe'', for example. This was essentially the same literary Scots which has been used by poets since the time of Allan Ramsay.

Dr David Purves,

8 Strathalmond Road, Edinburgh.

May 5.