The poetic device illustrated by the line is alliteration.
Alliteration is the poetic device illustrated by the line, '... all night long, by a fire-fly lamp.' Alliteration is the repetition of the same initial consonant sound in neighboring words or syllables. In this line, the repetition of the 'l' sound in 'long' and 'lamp' creates a musical and rhythmic effect, enhancing the imagery of the scene.
https://brainly.com/question/30521434