510 DIOPTRIC IMPERFECTIONS. [BOOK m. 



Not unfrequentlya radiate figure corresponding to the arrangement 

 of the fibres of the lens makes its appearance. 



Imperfections in the margin of the pupil appear in the shadow 

 of the iris which bounds the field of vision ; and the movements 

 of the iris in one eye may be rendered visible by looking at a bright 

 point or luminous surface through a pin-hole in a card placed close 

 in the front of the eye, in the anterior focus in fact, and then 

 alternately closing and opening the other eye ; the field of the 

 first may be observed to contract when light enters, and to expand 

 when the light is shut off from the second. The media of the eye are 

 fluorescent : a condition which favours the perception of the ultra- 

 violet rays. If a white sheet or white cloud be looked at in day- 

 light 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 the yellow spot. The prism must be frequently rotated, 

 as when the prism remains at rest the phenomena fade. Lastly, 

 the optical arrangements have a further imperfection in that the 

 dioptric surfaces are not truly centred on the optic axis. 



