If it really is a positive image on glass from the 1850s, the most likely process is an early collodion wet-plate image on glass, such as an ambrotype. That fits the period better than later dry plate processes. One community reply suggested tintype, but tintypes are made on metal rather than glass, so that would not match your description if the support is definitely glass. Identifying the exact process from a photo alone is difficult, so in-person examination or material analysis is the right next step. If you can confirm the image support, coating, and whether the image appears positive only against a dark backing, that would help distinguish among collodion glass processes and other early techniques.