This carte de visite photo was featured in Photo Sleuth in the spring 2017 edition of Military Images magazine. 
On 21 Aug 1862, James Merritt Wetherbee joined the 83rd Regiment, Illinois Volunteer Infantry, and served through the remainder of the Civil War.
During 1863, his regiment held Fort Donelson in Tennessee. 
According to Frederick Gaede , the “83rd was heavily engaged on February 3, 1863 at Fort Donelson, where it repulsed an attack by 8,000 Confederate troops under Joseph Wheeler and Nathan Bedford Forrest. During the engagement, known as the Battle of Dover, the regimental loss was 13 killed and 51 wounded. Soon thereafter, certainly by July, a number of the regiment were detached and ‘transferred to mounted Infantry to hunt guerrillas in 1863.’ It was reported in the Nashville Daily Union (Jan 1865) that Sergeant Brady of Wetherbee’s company and several other members of the 83rd captured Jake Sly and several companions who were ‘noted guerrillas.’”
The Photo Sleuth article describes Gaede’s identification of James’ weapons in the photo as a Merrill carbine and Starr revolver. Further, he says these weapons were primarily used by Union cavalry regiments, instead of infantry.  This suggests James was transferred to this mounted Infantry unit along with SGT Brady to hunt guerrillas.
“During the year 1864 the regiment had some 200 miles of communications to guard, as well as heavy patrol duty, and during the winter of 1864–65 it was on provost duty at Nashville, Tennessee.” 
He was discharged at the end of the war, on 5 Jul 1865, in Chicago.
James was born in Royalton, Niagara County, New York, the son of Ira Jay Wetherbee and Lydia Manchester. He removed to Illinois by 1857. After the Civil War, he removed to Fayette County, Iowa, by 1877, and to Palouse, Whitman County, Washington, by 1920. He was a farmer, and later a grain mercant/dealer.
He was married three times, his first two wives dying before the ages of 28 and 39 respectively. We have confirmed he had five children with his first wife Louisa Johnson, and six children with his third wife Catherine Maria Roberts.
According to the photo inscription, the photo was given to his first daughter, Ellen Rexaville Wetherbee.
PVT James Merritt Wetherbee (1831-1920) is 3rd great-grandson of John Witherby II (1677-1720), and 4th cousin 5x removed of MKS in the Wetherbee branch.
 Kurt Luther, Photo Sleuth—Merrill Carbine Leads to a Soldier’s Identification, Military Images, spring 2017.
 Wikipedia, “83rd Illinois Infantry Regiment.”