Note that William H. Jackson was one of the veterans in the picture posted in this blog post — 1938- Memorial Day, Gettysburg Veterans.

Jackson died on June 30, 1942 and the New York Times published:
Obituary (see col. 1)
Editorial (see col. 3-4)

Jackson’s Diary
Jackson wrote a diary during the Civil War, which is in the William Henry Jackson Papers (New York Public Library). The finding aid is available here.

Diary entries from Gettysburg not online, but were printed in William H. Jackson, Time Exposure: The Autobiography of William Henry Jackson, Profusely Illustrated with Photographs, Paintings, and Drawings by the Author (New York: Putnam’s, 1940), 66-67.

Part of his diary from 1869 is online

Text from finding aid — These “diaries” as a whole cover his nine months in the Vermont Regiment, 1862-1863; his first trip West in 1866-67; the opening of his studio in Omaha; his photography along the line of the Union Pacific Railroad in 1869; his “photographic campaigns” with the U.S. Geological Survey, 1870-1878; his travels abroad with the World’s Transportation Commission, 1894-1986; and his years in retirement, 1925-1942.

Other Online Image Collections
http://memory.loc.gov/ammem/wtc/wtchome.html
http://www.lib.byu.edu/dlib/jackson/
http://www.nps.gov/history/history/online_books/knudsen/index.htm

Posted by Don Sailer
Newspaper Credit – “W. H. Jackson Dies; Photographer, 99,” New York Times, July 1, 1942, p. 25: 1-2; “He Couldn’t Age,” New York Times, July 3, 1942, p. 25: 3-4. Historical Newspapers (ProQuest)