What's the difference between "mirror" and "looking glass"? [closed]

I have read that a looking glass is a surface with sufficient reflection to form an image of an object... doesn't that sound like a mirror?

There is no difference. "Looking glass" is a poetic and archaic way to refer to a mirror.

Looking glass was considered the 'proper' word to use when referring to what we now would all call a mirror. The word mirror was considered vulgar and middle-class by the upper classes. Some upper class people will still say looking glass instead of mirror. The word glass on its own also often refers to mirrors rather than glass. Hence pier glass, or 'go and look in the glass'.

When Perseus slays Medusa, he does so without looking directly at her, instead using his highly polished shield as a mirror. This sense of mirror is reflected (so to speak) in the first definition of the term in Merriam-Webster's Eleventh Collegiate Dictionary (2003):

mirror 1: a polished or smooth surface (as of glass) that forms images for reflection

But would it be correct to say that Perseus used his shield as a looking glass? The underlying question here is, How literally should we take the component "glass" in the term looking glass? If looking glasses must be made, in part, of glass, then a metal shield can't be a looking glass, though it can be a mirror.

The Eleventh Collegiate Dictionary isn't especially helpful on this point, as it defines looking glass simply as "MIRROR." Earlier dictionaries, however, don't treat the two terms as identical.

One of the clearest treatments of the differences between mirror and looking-glass appears in Merriam-Webster's [First] International Dictionary of the English Language (1890), which provides distinct definitions for three related terms:

Looking-glass A mirror made of glass on which has been placed a backing of some reflecting substance, as quicksilver.

Mirror A looking-glass or a speculum; any glass or polished substance that forms images by the reflection of rays of light.

Speculum 1. A mirror, or looking-glass; especially a metal mirror, as in Greek and Roman archaeology. 2. A reflector of polished metal, especially one used in reflecting telescopes. ...

From these definitions, it appears that (late in the nineteenth century, anyway) Perseus's shield would have qualified as both a mirror and a speculum, but not as a looking-glass.