Chap, hi.] SIGHT. 877 



source of an illumination placed in the plane of the principal 

 anterior focus, the shadows of objects in the vitreous humour 

 move in the same direction as the source of illumination. Hence, 

 by observing the direction of the movement of an entoptic image 

 resulting from the movement of the illumination, the position in 

 the eye of the object giving rise to the image may be determined, 

 regard of course always being had to the so-called mental inver- 

 sion of the retinal image. Stated more strictly the rule would 

 run thus. The shadows of objects in front of the nodal point 

 (§ 527) in the lens move in a direction contrary to and those of 

 objects behind the nodal point in the same direction as the move- 

 ment of the illumination ; moreover the more distant the object 

 from the nodal point the greater the movement of the shadow 

 caused by the same movement of the illumination. 



In this connection we may refer to one or two matters which 

 however cannot be called dioptric imperfections. 



If a white sheet or white cloud be looked at in daylight 

 through a Nicol's prism, a somewhat bright double cone or double 

 tuft, with the apices touching, of a faint blue colour, is seen in 

 the centre of the field of vision, crossed by a similar double cone 

 of a somewhat yellow darker colour. These are spoken of as 

 Haidinger's brushes ; they rotate as the prism is rotated, and are 

 supposed to be due to the unequal absorption of the polarized 

 light in that part of the retina which we shall study presently 

 as "the yellow spot." The prism must be frequently rotated, 

 since when the prism remains at rest the phenomena fade. We 

 may here remark that the media of the eye are fluorescent : a 

 condition which favours the perception of the ultraviolet rays. 

 There are other entoptic phenomena due to features of the retina, 

 of which we shall speak in treating of the development of visual 

 impulses. 



Lastly, returning to dioptric imperfections, we may add that 

 the optical arrangements are also to a certain extent imperfect in- 

 asmuch as the dioptric surfaces are, according to most observers, 

 not truly centred on the optic axis. 



