Scientists of course knew that the eyes were responsible but exactly what happened when light entered the eye and how the brain actually received the information was a mystery. The compound eye of Limulus has served as a model for studying vision since the early part of the century and before we describe some of that work it deserves an introduction. Arthropods, like Limulus, possess eyes, but the eyes can vary in complexity. Some are simple, consisting of a few photoreceptors, while others are large and can resolve images. As a result, the focus of the eye is fixed as it is part of the outer skeleton of the animal.
Some arthropods possess a type of eye known as a compound eye.
The compound eye is made of smaller, simple eye units, called ommatidia . Each ommatidia is composed of a cornea, which as was previously mentioned is formed from the outer exoskeleton. This cornea acts as a lens to focus light into the eye. It is produced by adjacent cells, usually four in number. The cone tapers to a receptor unit called a retinula which focuses the light into a translucent cylinder called the rhabdome.