THE EYE AS AN OPTICAL INSTRUMENT. . 223 



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, m 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 sensative 

 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 

 recoo-nised 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 sunlight 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 sensi- 

 tive 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 distance between the sensitive and the vascular layers 

 of the retina. It is done as follows : — 



If the focus of the light thrown on to the white of the eye 

 (the sclerotic) is moved slightly backwards and forwards, 

 tiie shadow of the blood-vessels and its image in the field 

 of vision will, of course, move also. The extent of these 

 movements can be easily measured, and from these data 

 Heinrich Miiller, of Wurzburg — whose too early loss to 

 science we still deplore — determined the distance between 

 the two foci, and found it exactly to equal the thickness 



