From the length of the cylinder it looks to in 32 S&W caliber. If so it is a Deringer 32 Caliber Pocket Model (That is the model name, est. 4000 made). 1874 to 1879. After Henry Deringer died in 1868, relatives tried to cash in on the name and for several years produced small pocket revolvers. Flayderman's 8A-050, Listed Good 150 Fine 375
Hope that helps