||The size and focal length of the mirror was not standardized for some time. Helmholtz had introduced a similar but smaller mirror for the examination of the retina.
This was tried by some aurists but deemed too small; others, in an attempt to catch more light, used huge mirrors and only gradually was a diameter of 6‑7 cm adopted.
The advantage of having a hole in the centre of the mirror is that it provides parallax‑free illumination, although even here some of the earlier writers recommended looking alongside the mirror, for binocular vision, aspect.
concentrated by means of a head mirror and shone directy into the ear canal.
Thus, it was not until the early 1920s that reliable small bulbs and dependable easily obtainable flashlight batteries made the otoscope, as we know it today, a practical proposition.