The final fancy dress of the Newport summer season of 1850 occurred on Wednesday, September 4th. It was covered by The Boston Herald on September 5th (“Grand Fancy Ball at Newport”, p. 4) and more extensively by The New York Herald on September 6th (“The Grand Fancy Dress Ball at Newport”, p. 1). The bulk of the coverage was devoted to lists of attendees and their costumes, as is typical for fancy dress balls, but there are some other tidbits of useful information as well. The New York Herald article is extremely lengthy, so I have not transcribed all of it. The article from The Boston Herald is quite short, but not nearly as interesting.
The ball was held at the rebuilt Ocean House, the original of which had opened in 1844, burned down, and been rebuilt. This Ocean House was not the same as the modern Ocean House in Newport. A different hotel by the same name opened in 1868, was demolished in 2005, and then rebuilt again in 2010.
At the RhodeTour website, Dr. Brian Knoth writes about the first two Ocean Houses, with specific mention of the 1850 Fancy Dress Ball:


