The eye

[kermit911]

Say it is dark outside. You have a little bit of light comeing in, VERY little. Your sitting in a room for example looking at a desk. You see somthing out of the corner of your eye, but when u look over to look at it, you can't see it, or you can't see it as clear as you did when u weren't looking at it directly. Why is it that you can make somthing out more clear in the dark with the side of your eye then looking at it directly?

Thanx all.

[jake-the-peg]

You eye has two types of receptors on the retina, cones and rods. Cones come in 3 flavours to sense the three primary colours, rods only sense black and white but are more sensitive.

Immediately opposite your pupil is the area of your retina where you sense the image from straight ahead - this is what you use when you are looking directly at something - it only contains cones.

This means that you have a better ability to sense objects under low light conditions in your peripheral vision.

Before CCD sensors Astronomers used this and, when using telesopes on faint objects would look slightly away from the object of interest - no, it's not easy and there is a knack to it.  

[NetSquirrel]

A shrewd observation, kermit.  The reason lies in the distribution of the light-sensitive cells lining your retinas: the "rods" and the "cones".  Rod cells are very sensitive to the level of light falling on them and are distributed all over the surface of the retina, except the centre part (called the fovea). Cone cells are less sensitive than the rods, but come in three types: those sensitive to long-, middle-, and short-wavelength radiation (often referred to as the red, green and blue cones respectively) allowing colour vision.  Almost all of the cones on the retina are located at the tiny fovea (the centre of vision), which is composed entirely of cones.  This allows for high-resolution vision and excellent colour perception of whatever you are looking directly at.  However, in low light conditions, the cones do not receive enough light to function, so the rods must take over.  This is why your vision seems blurred/fuzzy at night - the density of rods on the retina is very low compared to the density of cones at the fovea (similar to turning your desktop resolution down to 640x480, instead of 1600x1200).  And since there are no rods at the fovea at all, low-light sensitivity at the centre of vision becomes very poor.  So at night time, you can make out objects fairly well provided that the light from the objects is not focused onto your foveas, i.e. you can see things at night only if you do not look directly at them.

 

There is an excellent explanation here:

 http://mbb.harvard.edu/evolution_of_mind_and_brain/chapt er_2.pdf

[NetSquirrel]

I refreshed the page to make sure there weren't any answers about 5 seconds before I posted!  Sorry about the unnecessary drivel I've written - the link is good though.

[jake-the-peg]

 

The race is not always to the swift, nor the battle to the strong, but that's the way to bet.

[Calvesy]

In normal light, colour is perceived by 'cones'. These structures are most concentrated at the centre of vision, or fovea, which allows you to see amazing detail of distant objects. After a few minutes of darkness, there is not enough light to detect colours, so the 'rods' come into play to give you a monochrome view at very low light levels. Because of the greater priority in needing to identify distant objects during the day, there are very few rods occupying the fovea, which explains why you see dim objects more clearly at night by looking away from them.

[Clanad]

Since you've already been overwhelmed with knowledge, I would only add that the structures thoroughly explained also explain why you easily detect motion away from the center of the eye...

[El D]

Its standard practice among the armed forces at night to look just off centre to get a better picture.

[kermit911]

Thanx all, that answered it.  Thanx all