Capitals and Dashes
This poem shows off all of Emily Dickinson usual quirks.
For one, you've got her idiosyncratic use of capitalization, which basically means that she chose to capitalize whatever words she wanted, whenever she wanted. Take the first line for instance: "The Brain—is wider than the Sky— " (1). Both "Brain" and "Sky" are capitalized even though they aren't proper nouns.
Why would Dickinson break the rules of English with such flagrant disregard? It's anybody's guess. But to us Shmoopers, the capitalization here—and in many of her other poems—helps to really emphasize the chosen words (and therefore their connotations and themes). It also seems to heighten the words in a way. It accentuates the fact, for example, that we're not just talking about one brain here; we're talking about every human brain—and beyond that, the human capacity for thought. Yeah, that seems worth a capital letter to us.
Dash It
We've also got Dickinson's free use of the long dash in this poem. If you've read any of Dickinson's other poems, you've seen what a big fan this lady is of the long dash. It's kind of interesting in this poem in particular, because she has created something with such a perfectly manicured meter (check "Form and Meter" for more), but then she chooses to chop it all up with these dashes everywhere.
Who knows why she'd do such a thing. To us, it helps to accentuate certain parts of the poem. For example, when we read lines like "For—hold them—Blue to Blue—" each portion of the line seems to really stand out. It's like we're being asked to truly consider each grouping of words, and we just can't help but do what we're told.