Final answer:
The Army authorized the 'Half Chevron' with a green background, yellow trim, and a Caduceus to denote the rank of Hospital Steward in 1866.
Step-by-step explanation:
The rank of Hospital Steward in the Army was authorized to wear a 'Half Chevron' with a green background, yellow trim, and a Caduceus in the year 1866. This insignia was used to denote the rank of Hospital Steward, who were responsible for providing medical care in the Army.