0 a unit of measurement of the power of a lens, used for measuring a person's eyesight: --
A typical short-sighted person in Britain is -4.5 dioptres.
The individual's refractive power is then read off a linear dioptre scale.
I also understand that statistical surveys have shown that it is normal for adults to be slightly longsighted up to a power of about 1 dioptre.
The main difference between this amendment and my original one is that this amendment mentions four dioptres where my amendment mentioned five dioptres.
It seems too from the literature of one company operating in this country that 4 dioptres is the ceiling.
We took this into account in arriving at the limit of 4 dioptres for reading glasses.
There is some fraction of a dioptre difference in need of correction.
We adopted four dioptres because the professional advice was that that was a sensible cut-off point.
