Therefore, the rhodopsin, like the algae, reflects the light it does not absorb - which is generally the red wavelengths of light, causing its reddish color. Our own human rhodopsins are still red, a heirloom of our aquatic ancestry. The refractive index of a transparent substance is a measure of the relative speed of light in that medium. When light travels from one medium to another, such as passing from air into water, it slows down and changes direction. Everyone knows that objects in the water, when viewed from above the water, are not the same size or in the same place as they appear.
Corneal tissue has the same refractive index as water.
This means that the cornea of fish do not bend light as it enters the eye. In our eyes, light is slowed and bent as it moves through the cornea - the cornea acts like a lens to collect more light from a wider field of view. This means that the fish eye, already in a dark environment, will capture even less of the available light - less of the field of view. Without bulging pupils or alternately a rotational neck, fish would have a very limited visual field. It's not always beneficial for a fish to have a bulging periscopic eye.