The eye's retina receives and reacts to incoming light and sends signals to the brain, allowing us to see. There is, however, a part of the retina that doesn't give us visual information. This is our eye's blind spot. We can experiment the blind spot from the following activity.
We have to hold the card at eye level about an arm's length away. Close right eye and look directly at the cross with the left eye. We will notice the dot. But focus on the cross and be aware of the dot as we slowly bring the card toward our face. The dot will disappear, and then reappear, as we bring the card toward our face.
Now close left eye and look directly at the dot with your right eye. This time the cross will disappear and reappear as we bring the card slowly toward our face.
What is going on? Our optic nerves carry messages from our eye to the brain. This bundle of nerve fibers passes through one spot on the light sensitive lining, or retina, of our eyes. In this spot, our eye's retina has no light receptors. When we hold the card so that the light from the dot falls on this spot, we cannot see the dot.
A question remains why we don’t notice the blind spot in our day to day observation of the world? Now as a variation on this blind spot activity, we can draw a straight line across the card from one edge to the other, through the center of the cross and the dot. We will notice that when the dot disappears, the line appears to be continuous, without a gap where the dot used to be. So, our brain seems to automatically "fill in" the blind spot with a simple extrapolation of the image surrounding the blind spot. This is why you do not notice the blind spot.