The answer is to do with the biology of the eye. There are two types of
receptor cells in the eye and each has strengths and weaknesses:
1. Rod cells - only see black and white but are very sensitive (ie will
pick up very small amounts of light)
2. Cone cells - see color but are not as sensitive as rod cells.

At the point in the back of your eye (the retina) where your eye focuses
the light when you are looking directly at something there is an area
called the fovea. In the fovea the cells are very densely packed so that we
can see really well and because color is important to us the cells are
predominantly cone cells and only a very few rod cells. The problem with
this is that when the light is dim - the cone cells in the fovea are
actually really bad at picking up anything at all.

Outside the fovea there are more rod cells. Consequently in low light you
are better off not looking directly at an object because then the light
will fall on the (predominantly) rod cells outside the fovea rather than
the (predominantly) cone cells inside the fovea.



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