118 ANALYTIC SECT. XV. 
SECTION XV. 
Irritability of the Iris. 
“ Tta iris manifesto constringitur non propria vi que non sit 
irritabilis, sed a retine irritatione.”——HALLEr. 
ccLxxvill. The simplest subject becomes ob- 
scure, when involved in the double perplexity of 
sophistry and invalid facts. Haller, it appears from 
the quotation at the head of this article, relin- 
quished his irritability in the iris; but, by a 
curious perversity, he calls expansion of the iris 
contraction, to make its motion square with his 
theory of irritability. The errors of a great man 
find followers enough; the effects of what some 
Continental authors call tonicity, are in great 
part owing to expansibility. 
ccLxxrx. The colour of the iris has been alleged 
to proceed from thq pigmentum on its central 
aspect. In the inhabitants of the torrid zone, the 
iris is usually of a dark colour; it is whitish and 
almost transparent in the Albino; in cold coun- 
tries it is most commonly of a light hue. Some 
assert, that the colour of the eye-lashes and the 



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