36 LETTERS ON NATURAL MAGIC. 



minates ; but a ray thus falling obliquely on the 

 retina is seen, in virtue of the law of visible 

 direction already explained, in a line n C perpen- 

 dicular to the retina at the point near n, where 

 the ray fell. Hence the candle was necessarily 

 seen through the head as it were of the observer, 

 and without the range of ordinary vision. The 

 comparative brightness of the reflected image 

 still surprises me ; but even this, if the image 

 really was brighter, may be explained by the fact, 

 that it was formed on a part of the retina upon 

 which light had never before fallen, and which 

 may therefore be supposed to be more sensible, 

 than the parts of the membrane in constant use, 

 to luminous impressions. 



Independent of its interest as an example of 

 the marvellous in vision, the preceding fact may 

 be considered as a proof that the retina retains its 

 power to its very termination near the ciliary 

 processes, and that the law of visible direction 

 holds true even without the range of ordinary 

 vision. It is therefore possible that a reflecting 

 surface favourably placed on the outside of the 

 eye, or that a reflecting surface in the inside of 

 the eye, may cause a luminous image to fall 

 nearly on the extreme margin of the retina, the 

 consequence of which would be, that it would be 

 seen in the back of the head, half way between a 

 vertical and a horizontal line. 



