For an excellent history of the South Side of Waltham, see The South Side of Waltham written by Warren E. Glancy, 75 Riverview Avenue in 1974.
"The Island" was originally named "Morse Island", and was part of Newton. In 1849, Waltham purchased it (along with much of the South Side of Waltham) from Newton for $1,000.
In the 1890s, Morse Island was developed by the Crescent Park Land Association. This association was led by Daniel O'Hara, who founded the O'Hara Dial Company.
Many maps at that time reference The Island as "Crescent Park".