King Mongkut of Siam (r. 1851-1868), who is also known as Rama IV, was the fourth ruler of the Chakri Dynasty in what is now Thailand.
Mongkut was a modernizer and humanitarian. He also was the King during Anna Leonowens's visit to Siam, a stay that inspired her to write an autobiography later adapted into "The King and I."
Although the caption on this stereographic photo from the George Eastman Collection lists the date as 1875, obviously the photo was taken before that time, since King Mongkut passed away in 1868. His oldest son, seated to the king's right, appears to be no more than 10 years old in this photo, so it likely was taken around 1863.
That little boy succeeded to the throne at the age of 15 upon his father's death, and became King Chulalongkorn the Great (Rama V).