By mid-century, the stethoscope became an oft-used device and designs of the microscope had advanced enough for scientists to closely examine pathogens. The pioneering work of French microbiologist Louis Pasteur from the 1850s earned widespread acceptance for the germ theory of disease. It led to the introduction antiseptics by Joseph Lister in 1867 in the form of carbolic acid (phenol). He instructed the hospital staff to wear gloves and wash their hands, instruments, and dressings with a phenol solution and in 1869, he invented a machine that would spray carbolic acid in the operating theatre during surgery. Infection-related deaths fell noticeably as a result.