The fingernail test described above is a method for determining the difference between first-surface mirrors and second-surface mirrors, both of which are ordinary mirrors glazed in a different fashion. With a first-surface mirror, any object you put up against it will touch its reflection because the reflective part of mirror is laid in right at the surface. With a second-surface mirror, a gap between the object and its reflection will be visible because a layer of clear glass has been incorporated over the reflecti...

The fingernail test described above is a method for determining the difference between first-surface mirrors and second-surface mirrors, both of which are ordinary mirrors glazed in a different fashion. With a first-surface mirror, any object you put up against it will touch its reflection because the reflective part of mirror is laid in right at the surface. With a second-surface mirror, a gap between the object and its reflection will be visible because a layer of clear glass has been incorporated over the reflective part of the mirror to better protect it. First-surface looking glasses are quite expensive and are generally used only for specialized applications such as fine optical instruments or lasers, in which a protective layer of glass would interfere with the path of light or diminish precision. As such, you're unlikely to ever encounter one being used as an ordinary mirror in a home or a dressing room; for those applications much cheaper second-surface mirrors are nearly always used.