dioptre

But on average, the difference between dropping out at 16 and staying on until the end of university is "minus one dioptre".

Enough, we must not over-rate the value of the objective determination of the error of refraction, and I would estimate the limit of error at half a dioptre at least.

Opticians measure the ability of your eye to focus light in dioptres.

Minus one dioptre is relatively mild, but it's enough to need glasses for driving.

Isler found in hypermetropia of 2 to 10 dioptres squinting in 75 per cent.; in my statistics H. 1·5 D. to the highest degrees of hypermetropia are likewise represented by 75 per cent.