Of all our organs, the eye is probably the most astonishing. In an orbit of less than 3 cm3, we find a small mobile ball whose volume does not reach 7 milliliters, capable of informing us of tiny details as well as entire landscapes, of identifying the needle and in the next instant the haystack, of operating by the brightest lights on the snow in the sun, and by the most tenuous in the evening by candlelight.

All the colours, reflections and scintillations of light are accessible to him, and he succeeds in transferring them to our inner being at the very moment that they are produced or almost. It is through him that we acquire the great majority of the information necessary to govern our behaviour; through him that we form the majority of the inner images necessary for our understanding.

How can we not be astonished by the wonder of these two tiny balls whose volume is far from reaching a thousandth of our body but which could well provide the majority of its sensations?

So, looking around us at our fellow animals, how can we not wonder how their eyes work, these cousins, these brothers who have cohabited the earth with us for so long?

What do they see? Do they see the same thing as us? Do they see more? Do they see less? Do they see better or worse?

These difficult questions probably do not have a single, clear answer. At least we can try to clarify them and, failing to be able to put ourselves in the place of the animals, we can observe their eyes to try to guess the rest. This is the purpose of this book: an overview of animal vision.