Page:The wonders of optics (1869).djvu/45

 this point which differs for different eyes, you will find that if you diminish or increase the distance of the paper you will once more see the missing object. The same thing happens if you move the eye from the centre of the wafer. The same experiment may be repeated with the left eye with a precisely similar result.

It has been found by experiment that this particular blind space exists exactly over the base of the optic nerve, at the spot where it joins the eye. (Fig. 1.) Thus we see that the nerve which actually conveys the impression of sight to the brain is in itself incapable of being excited by light. In such cases as these Nature seems to laugh at us, and escapes from our grasp just as we are most confident in our power of wresting her secrets from her; indeed we may compare her to a wise and good-natured mother, who, though always amiable and willing to instruct those about her, sometimes smiles when her children fancy they are as learned as she is.

If we do not perceive the constant recurrence of the phenomenon just mentioned, it is because when both eyes are open the object whose image falls on the blind spot in one eye is seen by the other, the insensible portions of each eye being on opposite sides. Not only this: the spot being always situated on the outer and indistinct portion of the image reflected on the retina, we do not take notice of it; for as every one has no doubt observed, it is only the small portion of the object we are looking at exactly opposite the centre of the eye that is perfectly distinct and clear, all the rest being confused in its details, although quite visible.

Again, we may account for our not noticing it by the fact of our seeing clearly only those things which specially attract our attention—a fact first noticed by Mariotte. We see only what we wish to see with our physical eyes, as well as those of our mind. If our