If the receptors were at the front of the retina, the pigment cells would have to be between them and the next layer of nerve cells, in a region already packed with axons, dendrites, and synapses. As it is, the layers in front of the receptors are fairly transparent and probably do not blur the image much.
The definition of impervious is not being able to be penetrated or not able to be hurt. 1  An example of impervious is a fabric that does not allow any liquid to pass through. 2  An example of impervious is a figurine made out of indestructible material.
Most of the sentences you'll run across using impervious will be followed by the word to and a noun. Things are often described as being impervious to physical assaults like heat, water, bullets, weather, and attack, but just as frequently to less tangible things, like reason, criticism, pain, and pressure. The word comes from Latin: in- + pervius, meaning not letting things through..