A. G. Wetherby wrote to Christian Marinus Poulsen on 15 April 1879. The ink is pink. A bright, bubble-gum pink that I would not expect to see used by anyone over the age of twelve. I am somewhat surprised that the technology to make such an unnatural color existed in the nineteenth century. I suppose it might be a result of fading, over the years…but from what original color?
Also of note is the diaersis over the second “o” of Zoölogy (Wetherby was a professor of geology and zoology at the University of Cincinnati), a common rendering on letterhead. Wikipedia (sans citation) says the diaeresis was fading from English words by the 1940s. That feels late to me, though I can’t say why, and I certainly haven’t played extensively enough with early twentieth century documents to offer any solid impressionistic opinion. (And I’ve decided “solid impressionistic” is not an oxymoron.)