98 LETTERS ON NATUBAL MAGIC. 



natural agency becomes under ordinary circumstances 

 unavoidable. 



Hence it is not only an amusing but an useful occupa- 

 tion to acquire a knowledge of those causes which are 

 capable of producing so strange a belief, whether it arises 

 from the delusions which the mind practises upon itself, 

 or from the dexterity and science of others. I shall 

 therefore proceed to explain those illusions which have 

 their origin in the eye, whether they are general, or only 

 occasionally exhibited in particular persons, and under 

 particular circumstances. 



There are few persons aware that when they look with 

 one eye there is some particular object before them to which 

 they are absolutely blind. If we look with the right eye this 

 point is always about 15 to the right of the object which 

 we are viewing, or to the right of the axis of the eye or the 

 point of most distinct vision. If we look with the left .eye 

 the point is as far to the left. In order to be convinced oi 

 this curious fact, which was discovered by M. Mariotte, place 

 two coloured wafers upon a sheet of white paper at the 

 distance of three inches, and look at the left-hand wafer 

 with the right eye at the distance of about 11 or 12 inches, 

 taking care to keep the eye straight above the wafer, and 

 the line which joins the eyes parallel to the line which 

 joins the wafers. When this is done, and the left eye 

 closed, the right-hand wafer will no longer be visible. The 

 same effect will be produced if we close the right eye and. 

 look with the left eye at the right-hand wafer. When we 

 examine the retina to discover to what part of it this insensi- 

 bility, to light belongs, we find that the image of the in- 

 visible wafer has fallen on the base of the optic nerve, or the 

 place where this nerve enters the eye and expands itself to 

 form the retina. This point is shown in the preceding figure 

 by a convexity at the place where the nerve enters the eye. 



But though light of ordinary intensity makes no im- 



