54 ENTOPTIC PHENOMENA. [BOOK in. 



behind the nodal point in the same direction as the movement 

 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 polarised 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 

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

 not truly centred on the optic axis. 



