REMARKABLE OPTICAL ILLUSION. 119 



image of the candle cannot admit of a doubt; but the 

 wonder still remains how the images which it formed 

 occupied so mysterious a place as to be seen without 

 the range of vision, and apparently through the head. In 

 order to explain this, let m n, Fig. 2, be a lateral view of 

 the eye. The chip of wax was placed at m at the root of 

 the eyelashes, and being nearly in contact with the outer 

 surface of the cornea, the light of the candle which it re- 

 flected passed very obliquely through the pupil arid fell 

 upon the retina somewhere to the left of , very near 

 where the retina terminates; 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 re- 

 flected 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 mar- 

 vellous in vision, the preceding fact may be considered as 

 a proof that the retina retains its power to its very ter- 

 mination 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 woiild bo 

 that it would be seen in the back of the head half way 

 between a vertical and a horizontal line. 



