The ensuing Eye is a mixture of a simple eye within a compound eye. In the eyes of most mammals, birds, reptiles, and most other terrestrial vertebrates the vitreous fluid has a higher refractive index than the air. In refractive corneas, the lens tissue is corrected with inhomogeneous lens material , or with an aspheric shape. Flattening the lens has a disadvantage; the standard of imaginative and prescient is diminished away from the principle line of focus.