The Prince of Wales and Camilla, the Duchess of Cornwall, ended their second day in the U.S. with a glitzy night out with friends in Washington D.C. The couple, who will celebrate their 10th anniversary next month, put on their best dress for a dinner hosted by Barby Allbritton, the head of the prince's charity in the U.S. and the widow of Joe Allbritton, who owned news stations and the website Politico. The Allbrittons were so close to the British royals that they were two of a few Americans invited to the Royal Wedding between Charles' son Prince William and Kate Middleton in 2011. Charles and Camilla joined Mrs Allbritton, her son Robert - who now runs Politico - and his wife Elena for the black tie charity affair on Wednesday night. The event was so secretive that other guests were only given the address earlier in the day, PageSix reported. Prince Charles and Camila, Duchess of Cornwall, smile outside the a private home on Wednesday where they had dinner. From left: Elena Allbritton, Robert Allbritton, the Duchess of Cornwall, Barby Allbritton and Prince Charles . The Duchess of Cornwall made a splash at the dinner wearing an all-beige outfit complete with a cape . Barby Allbritton (second right), is the head of Prince Charles' charities in America. She is the wife of the late media magnate Joe Allbritton, who owned news stations and the website Politico . The night out came in the middle of the royal couple's four-day trip to the U.S., which is largely being spent in Washington D.C. before a trip to Louisville, Kentucky on Friday. Charles and Camilla still look bright-eyed as they arrived at the party despite a whirlwind day  visiting D.C.'s landmarks and George Washington's plantation before delving into environmental policy. At a meeting of the Global Ocean Commission and the International Sustainability Unit - a charitable group established by the prince to address environmental concerns - Charles spoke about the dangers of accumulating plastic waste in the world's oceans. 'Although the evidence surrounding the links between plastic waste and human health are not yet clear, I would tentatively suggest that they are unlikely to be beneficial,'he said during his 10-minute speech at the Hay Adams hotel. Charles said that he was ‘haunted’ and ‘horrified’ by the issue which needed to be solved for the sake of future generations. Passionate: Following a day of sight-seeing on Wednesday, Prince Charles also attended an environmental event at the Hay Adams hotel, where he gave a 10-minute speech on the importance of recycling . Meeting the King: Charles and Camilla arrive at the Martin Luther King Memorial in Washington DC, where they were met by Congresswoman Terri Sewel, left, and civil rights leader Jesse Jackson, right . Making friends: The royal couple drew a crowd as they arrived at the Memorial on Tuesday . Fans: Members of the public, including a middle school history class, snap photos of the royal couple . ‘Speaking as a grandfather with a new grandchild due to appear in this world in a month’s time, I think we probably owe it to everyone else’s grandchildren to grasp that solution,’ he said. His impassioned speech came after he and his wife took time out to play tourists. On Wednesday morning, the couple wrapped up against the brisk temperatures for their 20-minute tour of the Martin Luther King memorial and paused to read his quotations engraved in the stone. 'Darkness cannot drive out darkness, only light can do that. Hate cannot drive out hate, only love can do that,' read one quotation where the royal couple paused to see King's words from 1963. Harry Johnson, the president of the foundation that built the King memorial, said he gave the royal couple a stone from the memorial's granite as a gift. The couple then visited the Lincoln Memorial, where they were greeted by historians. Towering: The royals posed in front of the memorials with members of congress and civil rights leaders . Paying their respects: Charles and Camilla speak with Ed Jackson during their tour of the memorial . Taking the tour: Camilla, who was chic in a grey coat, was shown around by Congresswoman Terri Sewel . During the visit, they were happy to chat with students on field trips. Joan Darby, who brought her U.S. history class from Stafford, Virginia, to the King memorial said they were thrilled to meet Charles and Camilla, along with civil rights leaders Rev. Jesse Jackson and Rep. John Lewis. 'To come here and have such a joyful event was just really incredible luck on our part,' she said. Camilla, who was showing off a chic new hairstyle, obligingly stopped for a selfie before continuing on with U.S. Rep. Terri Sewell by her side. Charles and Camilla were then whisked off to Mount Vernon, where they were met by the Governor and First Lady of Virginia, Terry and Dorothy McAuliffe. One of the most historic spots in the US, parts of the estate have been in the Washington family since the mid-17th century and it was at Mount Vernon that George Washington, the first president of the United States, lived. Spectacular: The white marble monument is arguably the most famous in Washington D.C . Spotted: The royal couple were spotted by a party of American schoolchildren and swiftly surrounded . Pleased: Prince Charles appeared delighted to see so many well-wishers outside the Memorial . Obliging: Camilla even agreed to join one excited teenager for a royal selfie before moving to the next stop . Excited: News of the royal visit brought a crowd of excited well-wishers onto the street . Prince Charles, who arrived in the U.S. yesterday evening with his wife, also visited the National Archives today  to view the 1297 Magna Carta and the Declaration of Independence. The document, a successor to the original 1209 charter, was reissued by Edward I as a quid pro quo for extra funds with which to wage war on Scotland. It was also on a tour of the National Archives that Charles learned his uncle once developed a new polo stick. National Archivist David Ferriero presented the prince with a patent application submitted in 1931 by his uncle, Lord Louis Mountbatten, for a new kind of polo stick. Charles, himself a polo player, laughed and said he had no idea that his uncle had designed a polo stick. He also laughed when shown a telegram from the U.S. embassy in London to the Secretary of State, sent in 1957 seeking instructions to ensure that a toy car that had been given to Charles, then 8 years old, would be working properly when he returned home to play with it. Camilla, meanwhile, visited the Shakespeare Theatre Company. During an afternoon visit to the National Archives, Prince Charles laughs as he's shown a telegram from the American Embassy in London to the U.S. Secretary of State in 1957, asking for instructions on how to operate a toy car that was to be gifted to the then 8-year-old prince . Above, the telegram from the U.S. Embassy on the left and on the right, a patent by Prince Charles uncle Louis Mountbatten for a new design of polo stick, which the prince viewed at the National Archives on Wednesday . Prince Charles also viewed copies of the Declaration of Independence and Magna Carta during his trip to the National Archives on Wednesday . Pleasure: Camilla greets performers during a visit to the Shakespeare Theatre Company . Warm reception:  Camilla Duchess of Cornwall . Prince recieves flowers from a student at the Shakespeare Theatre Company and the Harman Center for the Arts . On Thursday, they will meet with President Obama. The couple flew into the U.S. Tuesday night, arriving at Andrews Air Force Base in Washington DC on a private plane. The royal tour has been welcomed by British ambassador Sir Peter Westmacott, who hopes the visit will give a boost to Anglo-American relations. 'I am so pleased to welcome The Prince of Wales and The Duchess of Cornwall back to the United States, ten years after their first official visit together,' he said. 'Their trip to Washington and Kentucky will showcase the best of the transatlantic relationship - promoting democratic values, combating climate change, preserving the natural environment, creating opportunities for the next generation - and provide yet another reminder of the essential role the Royal Family play in keeping that relationship special.' Charles and Camilla will also visit Louisville in Kentucky. Next stop: The couple then toured the plantation home of George Washington in Mount Vernon, Virginia . Charles and Camilla walk down a colonnade during their visit to Mount Vernon as visitors snap photos . Making friends: The Duchess of Cornwall greets Gunner, an Airedale Terrier, during her visit to Mount Vernon . TUESDAY . The couple landed at Andrews air force base in Maryland and attended a reception at UK Ambassador's residence in Washington . WEDNESDAY . Charles will mark the 800th anniversary of Magna Carta during a visit to the National Archives where he will view the 'Charters of Freedom' and the 1297 version of the Magna Carta . Charles will give a speech at the Hay-Adams Hotel in Washington to representatives of major companies, leading environmental organisations and government gathered together to look at the issue of plastic waste in seas and oceans . Camilla will visit the Shakespeare Theatre Company where she will tour the theatre, meet staff and children who use the theatre and then view a short production . THURSDAY . Charles will visit literacy and cookery classes at the Carlos Rosario Public Charter School, which provides education and training for adult immigrants in Washington. The couple will have talks with US President Barack Obama and his Vice President Joe Biden at the Oval office. FRIDAY . The couple will tour Louisville in Kentucky, which features a cultural centre dedicated to the life and achievements of three-time former heavyweight boxing champion Muhammad Ali. They will also attend a cultural festival hosted by the Governor of Kentucky and Mayor of Louisville. Camilla will visit Neighborhood House, a community centre serving one of Louisville's poorest communities. She will also be at a reception for patrons and supporters of animal welfare organisation The Brooke USA at Churchill Downs.
Charles and Camilla attended a dinner hosted by Barby Allbritton, the head of the prince's charity in the U.S., on Wednesday night . The royal couple are on a four-day trip to the U.S. which will mostly be spent in Washington D.C. before a visit to Louisville, Kentucky . On Wednesday morning, they were swarmed by well-wishers as they visited the Martin Luther King and Abraham Lincoln Memorials in D.C. They then headed to George Washington's Virginia home before returning to D.C., where Charles gave a passionate speech about the environment . On Thursday, the royal couple will meet with President Obama .