SIGHT. 569 



thing to which the mind attends, and not the picture on the 

 retina. The mind is not placed behind the eye to look at this 

 image, as an observer is placed behind a camera obscura : 

 neither does it in this case, more than in any other concerning 

 perception, attend to the sensations themselves, which serve solely 

 to suggest to it the causes which produce them. 



" Since many conditions are requisite for distinct vision, the 

 Creator has wonderfully ordered the functions of these organs. 



" A sufficient, but, at the same time, a definite, quantity of 

 light, not too intense for distinct vision, is provided in two 

 modes: First, according to the greater or less intensity of 

 the rays, a greater or less number of them pass to the lens ; 

 Secondly, that portion which is superabundant and injurious to 

 vision is absorbed. 



" The first point is effected by the motion of the iris ; the se- 

 cond, by the pigmentum nigrum. 



" The iris is endowed with remarkable mobility, and thus ac- 

 commodates itself to the intensity and distance of light, so that, 

 when exposed to a strong light or to near objects, it may ex- 

 pand itself and contract the pupil, but, when to a weaker light 

 or more remote objects, it may contract itself and dilate that 

 opening. 11 



" Physiologists have given different explanations of this mo- 

 tion. Some ascribe it to the varied impulses of blood into the 

 vessels ; others to contraction of the imaginary muscular fibres 

 of the iris. I have shown, in a particular treatise, that both 

 these circumstances are impossible, and that its proximate cause 

 may be sought for with more probability and reason in the 

 vita propria of the iris ; the more remote cause, as we formerly 

 hinted, can be solely the reaction of the sensorium. * 



" The function of the dark pigment, so frequently mentioned, 

 viz. to absorb the superfluous rays, and, consequently, its im- 

 portance 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 



h " Zinn, De Motu Uvece, 1757, in the Comment. Soc. Sclent. Gotting. t. i. 

 Fel. Fontana, Dei Moti ddV Iride. Lucca. 1765. 8vo." 

 1 " For other explanations consult Troxler in Hiraly's Ophthalmol. Biblioth. t i. 

 P. ii. p. 21." 



