12 LETTEES ON NATURAL MAGIC. 



with the left eye, the point is as far to the left, 

 In order to be convinced of 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 invisible 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 impression upon this part of the eye, a very 

 strong light does, and even when we use candles 

 or highly luminous bodies in place of wafers, 

 the body does not wholly disappear, but leaves 

 behind a faint cloudy light, without, however, 

 giving anything like an image of the object from 

 which the light proceeds. 



When the objects are white wafers upon a black 

 ground, the white wafer absolutely disappears, 

 and the space which it covers appears to be com- 

 pletely black ; and as the light which illuminates 

 a landscape is not much different from that of a 



