Laudator Temporis Acti

Charles C. Calhoun, Longfellow: A Rediscovered Life (Boston: Beacon Press, 2004), p. 140:

He got the idea from a scrap of a New York newspaper that bore the state seal, a rising sun with the motto "Excelsior," a word of dubious Latinity as used in his poem, translated as "higher."

Mortimer Collins, Thoughts in My Garden, Vol. I (London: Richard Bentley and Son, 1880), pp. 132-133 (August 24, 1871):

The other day I had the pleasure of discussing with an American gentleman the quality of Longfellow's poem 'Excelsior.' I maintained that, beyond the inherent absurdity of any young person's trying to climb the Matterhorn late at night, without guides, and with a banner in his hand instead of an alpenstock, the Latin refrain of the poem could only mean that the youth in question grew taller as he went farther up. Longfellow ought to have written 'Excelsius,' if he meant higher up—as there seems reason to suppose. My American was quite aggrieved, and remarked that the word had been accepted in the United States—that ships were christened 'Excelsior'—that ladies' schools were called 'Excelsior Seminary,' and the like—that there was at least one 'Excelsior Terrace' in every town in the great Republic. 'That,' he said, 'is sound fame'—and I have no doubt Mr. Hepworth Dixon would agree with him. 'But,' I retorted, 'it is unsound grammar.' 'What do we Americans want of Latin grammar?' he asked, indignantly. 'Nothing at all,' was my reply; 'nobody asked you to use Latin, but if you will do it, you might use it accurately.'

D.B. Wyndham Lewis and Charles Lee, The Stuffed Owl: An Anthology of Bad Verse (1930; rpt. New York: New York Review Books, 2003), p. 189:

A bitter controversy has raged round the young man's cry of "Excelsior!" One school of philologists holds that he meant to cry "Excelsius!" the comparative of the adverb excelsè, but that what with the late hour and the severe Alpine cold and (as Miss Beatrice Lillie would say) what with this and what with that, he confused it with the comparative of the adjective or past participle excelsus. Arrayed in opposition is a school which insists that the banner with a strange device and the young man's odd behaviour altogether were deliberately designed by Longfellow, the seer, to foreshadow the more striking developments of American Publicity, and that "Excelsior!" therefore is to be regarded as perfectly good Big Business Latin.

Longfellow, letter to Vincenzo Cesati (February 5, 1874):

I have had the pleasure of receiving your card with your friendly criticism on the word "Excelsior."

In reply I would say by way of explanation that the device on the banner is not to be interpreted "ascende superius" but "scopus meus excelsior est."

This will make evident why I say "Excelsior" and not "Excelsius."

Longfellow, letter to Allen Page Moor (April 14, 1876):

What you say of the word "Excelsior" is doubtless right from your point of view, but not from mine. You take it for granted that I meant St. Luke's "Amice, ascende superius." No, that is not my meaning, but rather "Scopus meus excelsior est"; and with this interpretation of the broken phrase, the adjective is right, and not the adverb. And I prefer this interpretation to the other, because Excelsior is a much more sonorous ending to the stanza than Excelsius would be.

Longfellow, letter to Curtis Guild (April 18, 1876):

I know that critics say the word should be excelsius, and they are right if they understand the broken phrase in the scriptural sense of Amice, ascende superius. I do not so fill up the sentence but with some such words as Scopus meus excelsior est. Then the adjective is right and not the adverb.

And this interpretation I prefer, because excelsior is a better word for my purpose than excelsius, having a more sonorous termination.

The Latin word for goal used by Longfellow in his explanation, scopus, is rare (borrowed from Greek σκοπός). More usual words are meta, finis, or terminus.