Why did doctors wear metal disks with holes in them on their heads?
In days of old, doctors wore metal disks with a hole in the middle on their heads, which made them look like a coal miner or a shaman. What was the disk and where did it go?
Cecil has heard various terms for this, but the simplest, most descriptive, and therefore most unmedical is "head mirror." It was used in examinations of the ear, throat, and other, ah, body cavities. To use, you swung the head mirror down so that you could look through the hole in the middle with one eye. Then you positioned a light source so that it shone on the mirror's parabolic surface. By moving your head just so, you could reflect the light rays down the patient's throat or whatever, the better to illuminate items of interest without obstructing the view.
Just about all doctors used head mirrors at one time and they became, along with the stethoscope, one of the symbols of the profession. But they could be a bit of a hassle to use and they did make you look like a space alien, so today many doctors prefer a penlight or other examining device. Some ENT (ear-nose-throat) specialists still use head mirrors, though, so look one up if you get nostalgic.