THE EYE AS AN OPTICAL INSTRUMENT. 197 



vision. The way to recognise this blind spot most readily is 

 doubtless known to many of my readers. Take a sheet of white 

 paper and mark on it a little cross ; then to the right of this, on 

 the same level, and about three inches off, draw a round black 

 spot half an inch in diameter. Now, holding the paper at arm's 

 length, shut the left eye, fix the right upon the cross, and bring 

 the paper gradually nearer. When it is about eleven inches 

 from the eye, the black spot will suddenly disappear, and will 

 again come into sight as the paper is moved nearer. 



This blind spot is so large that it might prevent our seeing 

 eleven full moons if placed side by side, or a man's face at a 

 distance of only six or seven feet. Mariotte, 1 who discovered 

 the phenomenon, amused Charles II. and his courtiers by 

 showing them how they might see each other with their heads 

 cut off. 



There are, in addition, a number of smaller gaps in the field 

 of vision, in which a small bright point, a fixed star for example, 

 may be lost. These are caused by the blood-vessels of the retina. 

 The vessels run in the front layers, and so cast their shadow on 

 the part of the sensitive mosaic which lies behind them. The 

 larger ones shut off the light from reaching the rods and cones 

 altogether, the more slender at least limit its amount. 



These splits in the picture presented by the eye may be re- 

 cognised by making a hole in a card with a fine needle, and 

 looking through it at the sky, moving the card a little from side 

 to side all the time. A still better experiment is to throw sun- 

 light through a small lens upon the white of the eye at the 

 outer angle (temporal canthus), while the globe is turned as 

 much as possible inwards. The shadow of the blood-vessels is 

 then thrown across on to the inner wall of the retina, and we 

 see them as gigantic branching lines, like fig. 32 magnified. 

 These vessels lie in the front layer of the retina itself, and, of 

 course, their shadow can only be seen when it falls on the 

 proper sensitive layer. So that this phenomenon furnishes a 

 proof that the hindmost layer is that which is sensitive to light. 

 And by its help it has become possible actually to measure the 

 1 Edme. Mariotte, born in Burgundy, died at Paris, 1684. 



