If we can rule out fairly large blown-glass spherical mirrors, that would seem to leave only small polished metal mirrors, which were known in the 15th century and indeed earlier.

• 如果排除掉大型玻璃球镜的可能性，就剩下较小的金属磨光镜；而这种镜子的确在15世纪或更早之前就已经存在。