I was not pulling anybody’s leg if I said that in our field of vision we have a section we fail to see at all, though it is right before our noses. Indeed nobody ever notices in his or her lifetime such a signal defect. Okay, if you don’t believe me, the pic above is a simple experiment to show that there really is such a thing!
See the picture above? Position your face very close to your computer screen, meanwhile cupping or covering your right eye, and look at the black circle on the right. Gradually and gently move your face away from the screen (with your left eye still focusing on the larger black circle on the right). At one moment the smaller black circle will vanish without trace . You won’t see it, though it is still within the limits of your field of vision. But if you distract the focus from the black circle on the right to meet the smaller black circle on the left, it will return to your sight. As soon as you focus back on the right black circle, the one on the left will again ‘disappear’!
Now, let’s switch the object. Position your face again close to your computer screen, this time you cup your left eye and focus on the cross (with the smaller black circle) at the left. Again, gradually move your face away from the screen. This time, at one moment the large black circle at the conjunction of the two white circles will vanish “into thin air”. It is still in the range of your field of vision but you fail to see it! Even you will see that the two white circles at the right will be quite distinct!
This phenomenon was first observed by a Frenchman, Edme Mariotte in the 17th century. The area where you fail to see an object even it is in the range of your field of vision is called the “blind spot” or scotoma. This is the area where the optic nerve enters and has no sensitive visual cells. We never notice the presence of this blind spot because we have been living with it since birth thus we have grown accustomed to it. Do you wear glasses? Good! Then you might like to try the following experiment. Glue a scrap of paper to one of the lenses, only not quite dead centre. For a few days (a week at longest) it will be a nuisance, but after approximately a week you will even cease to notice it at all! Believe me! 😉 Or if you are rich enough or you care to bother yourself by cracking your own glasses (don’t break them just crack them) you will notice that the crack will annoy you at the beginning. After a few days you will even forget to buy new glasses until someone tells you to do so. Yes, it is a matter of habit that is responsible for our being “blind” to our blind spot. Then one must note that the blind spots of either eyes correspond to different places in the two fields of vision. Consequently, with two-eyed vision, there is no deficiency in the range they both cover. It even makes more sense to why we are always “blind” to our own blind spot because mostly we see everything around us without an eye patch! 😉
If you want to see another experiment on the blind spot you can beam yourself (StarTrek mode: ON) onto the Wiki’s page about blind spot.