angle-worm : think of anglers and the bait they use
raw : emphasises the wildness of the bird
from a grass : makes the action almost resemble the human action of drinking from a glass
beads/velvet : the speaker describes the bird in terms of civilisation
plashless : a less common form of splashless
Form
Written in five four-lined verses called quatrains (Dickinson uses iambic trimeter with occasional four-syllable lines).
Tone
In Stanzas 1 – 3 the poet develops a sense of surprise, curiosity and even amusement at the bird’s behaviour.
The last two stanzas the tone changes to one of awe and wonder at the magical beauty of the bird in flight.
Rhyme and rhythm
A loose ABCB rhyme scheme.
Language and imagery
The most remarkable feature of this poem is the imagery of its final stanza, in which Dickinson provides a breath-taking description of flying. Simply by offering two quick comparisons of flight and by using descriptions that would normally be associated with movement in water - rowing and swimming - she suggests the delicacy and fluidity of moving through air. The image of butterflies leaping "off Banks of Noon", splashlessly swimming through the sky, is a memorable one.
Dickinson used common language in startling ways - a strategy called ‘defamiliarization’.