Edward Curtis letter re: autopsy

[Marty L.]

I am an academic working on an article about the constitutionality of the assassins' military commission. I have a discrete question that perhaps someone here can answer: I've read--and would like to quote--the famous excerpt from Edward Curtis's remarkable letter to his mother regarding the Lincoln autopsy, but I can't seem to find any citation to it, or information about where it can be found.

Thanks in advance for any information.