OF SIGHT. 175 



of them pass to the lens; Secondly, that portion 

 which is superabundant and injurious to vision, is 

 absorbed. 



The first is effected by the motion of the iris ; the 

 second, by the pigmentum nigrum. 



273. The iris is endowed with sufficient mobility to 

 accommodate itself to the intensity and distance of 

 light, that, when exposed to a strong light or to near 

 objects it may expand itself and contract the pupil, 

 but when to a weaker light or more remote objects it 

 may contract itself and dilate that opening.* Physio- 

 logists have given different explanations of this motion. 

 Some ascribe it to the varied impulse of blood into its 

 vessels; others to contraction of its imaginary muscular 

 fibres. I have shewn, in a particular treatise, that 

 both these circumstances are impossible, and that its 

 proximate cause may be sought for with more proba- 

 bility and reason in the vita propria of the iris (42); 

 the more remote cause, as we formerly hinted (56), can 

 be solely the reaction of the sensorium.f 



274. The function of the dark pigment, so frequently 

 mentioned, (258, 261, 263,) viz. to absorb the super- 

 fluous rays, and its importance to the perfection of 

 vision, are demonstrated, among other modes, by the 

 dissection of different kinds of animals, and by the 

 diseased condition of Albinos, whose eyes are very 

 tender and impatient of light from the absence of this 

 pigment. 



* Zinn, De Motu Uveee. 1757. in the Comment. Societ. Scient. dotting: T. i. 



Fel. Fontana, Dei Moti deW Iride. Lucca. 1765. 8vo. 



f For other explanations consult Troxler in Himly's Ophtkalmol. Biblioth. 

 T. i. P. ii. p. 21. 



t I have spoken of Albinos at large in my work De Generit Humani Va- 

 rietate Nativa. ed. 3. p. 274 ; and in my dissertation De Octtlis Lcucestfriopitm. 



