PROTECTION OF THE EYE. 309 



tense illumination of the object upon which it is 

 directed. 



The iris is a curtain, or septum, which stretches 

 across the aqueous humour, and is anterior to the 

 crystalline lens : it is perforated in the centre, and 

 that perforation is the pupil — the black central 

 spot which we see when we look into the eye — 

 black, for the reason which we have assigned, 

 that the rays piercing there are not returned, and 

 the absence of rays is blackness. But the rays 

 strike the anterior part of the iris itself round the 

 perforation, and they are partially, at least, re- 

 flected, giving the colour to the eye — gray, or 

 blue, or hazel. 



Perhaps the diagram (vol. i. pp. 24, 25) will 

 explain the structure of the iris and pupil ; and 

 show the mode in which the pencil of rays is en- 

 larged or diminished, and the intensity of the 

 image in the eye thus made greater or less in 

 proportion to the illumination. 



The iris, then, we understand to be a muscular 

 septum or partition, with two sets of fibres ; a 

 straight set converging to that margin of the iris 

 which forms the pupil, and a circular set running 

 round the exterior margin of the iris. At page 

 250 of vol. i. we have given a representation of 

 the iris of the lion. The pupil or open space is 

 oval in this animal ; b b are the straight fibres 

 converging to the exterior margin of the iris ; and 

 c c are the circular fibres of the margin. These 

 two sets of fibres act against each other, and in a 



