Not uniformly higher, but a gradient of refractive index such that the spherical region functioned as an excellent graded- index lens. Humans make lenses by grinding glass to a particular shape. We make a compound lens. A graded-index lens, by contrast, has a continuously varying refractive index with in its own substance. Typically, it has a high refractive index near the centre of the lens.
Fish eyes have graded-index lenses.
This ratio is called Mattiessen's ratio. Nilsson and Pelger's computer model homed in unerringly on Mattiessen's ratio. And so to the question of how long all this evolutionary change might have taken. In order to answer this, Nilsson and Pelger had to make some assumptions about genetics in natural populations. Heritability is a measure of how far variation is governed by heredity.