Who is the "great American" in Martin Luther King's speech?

Just after welcoming the crowd in his "I Have a Dream" speech, Dr. Martin
Luther King, Jr. delivers this line:

Five score years ago
a great American in whose symbolic shadow we stand today signed the Emancipation
Proclamation.

This "great American" is a
reference to former President Abraham Lincoln, who signed the Emancipation Proclamation on
January 1, 1863. Lincoln said that this effort to free all slaves from their places of bondage
was the right move and noted that he never felt more certain that I was doing right, than I do
in signing this paper. Lincoln later became known as the Great Emancipator.

King alludes to Lincoln's efforts to grant all slaves the freedom they deserved in
order to share another truth about African Americans living in August 1963. In many ways, they
remained enslaved:

But 100 years later the Negro still is
not free. One hundred years later the life of the Negro is still badly crippled by the manacles
of segregation and the chains of discrimination.

King...