Buttons - The button and configuration designate the rank and branch of service of the officer.
18...For General Officers and Officers of the General Staff - gilt, convex, with spread eagle and stars, and plain border; large size, seven - eighths of an inch in exterior diameter; small size one - half inch.
19...For Officers of the Corps of Engineers - gilt, nine - tenths of an inch in exterior diameter, slightly convex; a. raised bright rim, one - thirtieth of an inch wide; device, an eagle holding in his beak a scroll, with the word "Essayons," a bastion with embrasures in the distance surrounded by water, with a rising sun - the figures to be of dead gold upon a bright field. Small buttons of the same form and device, and fifty - five hundredths of an inch in exterior diameter.
20...States, occupying one - half the diameter, and the letters T. E. in old English characters the other half; small buttons, one - half inch diameter, device and form the same.
21...For Officers of the Ordnance Department - gilt, convex, plain border, cross cannon and bombshell, with a circular scroll over and across the cannon, containing the words “Ordnance Corps:” large size, seven - eighths of an inch in exterior diameter; small size, one - half inch.
22...For Officers of Artillery, Infantry, Riflemen, Cavalry, and Dragoons - gilt, convex; device, a spread eagle with the letter A, for Artillery - 1, for Infantry - R, for Riflemen - C, for Cavalry - D, for Dragoons, on the shield; large size, seven - eighths of an inch in exterior diameter; small size,
one - half inch.
23...Aides - de - camp may wear the button of the General Staff, or of their regiment or corps, at their option.  
24...For all Enlisted Men - yellow, the same as is used by the Artillery, &c., omitting the letter in the shield.
                                Description from revised regulations of 1861 

Button Configuration -

2...For a Major General–two rows of buttons on the breast, nine in each row, placed by threes ; the distance between each row, five and one - half inches at top, and three and one - half inches at bottom
3...For a Brigadier General - the same as for a Major General, except that there will be only eight buttons in each row on the breast, placed in pairs.
4...For a Colonel - the same as for a Major General, except that there will be only seven buttons in each row on the breast, placed at equal distances ; collar and cuffs of the same color and material as the coat.  
5...For a Lieutenant Colonel - the same as for a Colonel.
6...For a Major - the same as for a Colonel.
7...For a Captain - the same as for a Colonel, except that there will be only one row of nine buttons on the breast, placed at equal distances.
8...For a First Lieutenant - the same as for a Captain.
9...For a Second Lieutenant - the same as for a Captain.
10...For a Brevet Second Lieutenant - the same as for a Captain. 

Description from revised regulations of 1861