сияние -> (П. п.) сиянии or сиянье

Studying Lermontov's poem "Выхожу один я на дорогу" I find "Спет земля в сиянье голубом" and not "в сиянии голубом" as could be waited. I have translated it "The Earth sleeps in a bluish shining" (more or less). Looking сиянье in Морфологический анализ, it is not refused, however it is not a included form of the word сияние . I have found in other songs the same нь where it shouldn't be expectated. For example in "Очи чёрные", "Где страданья нет" and not "Где страдания нет". And also is was not refused by Морфологический анализ. It is only a license in songs to express more emphatically the softness of a, in any case, soft consonant or it is rather an archaic form (previous to the reforms of the years 1920) ? Thank you.

There are also "поздравленье" and "день рожденья" that are widely used and not archaic at all. :) I'd rather say they are more colloquial than "поздравление" and "день рождения" and they are used in everyday life, not only in lyrics.

Thank you.

Не за что. :)

We use «нь» instead of «ни» just to shorten words and ease of speaking. Otherwise these words are synonyms. I would say that children especially like the «нь» forms.

In your example (Lermontov's poem) the «сиянии» instead of «сиянье» will break the rhythm.

Ладно. Спасибо.