The results were swift and decisive. A trajectory of steadily mounting acuity led unhesitatingly from the flat beginning through a shallow indentation to a steadily deepening cup, as the shape of the model eye deformed itself on the computer screen. The transparent layer thickened to fill the cup and smoothly bulged its outer surface in a curve. And then, almost like a conjuring trick, a portion of this transparent filling condensed into a local, spherical subregion of higher refractive index. 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.