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.
The imperfect (abbreviated IMPERF) is a verb form, found in various languages, which combines past tense (reference to a past time) and imperfective aspect (reference to a continuing or repeated event or state). It can therefore have meanings similar to the English was walking or used to walk..