186 PHILOSOPHY OF ZOOLOGY. 



both, and thus alter the form of the humours, or change 

 their relative position with regard to the retina. 



The crystalline lens has also been regarded as possessing 

 the power of changing its form, and varying the focal dis- 

 tance of the eye, either in consequence of the action of the 

 ciliary processes, or of a change in the internal arrangement 

 of its parts. Its structure, as displayed by LEUWENHOEK *, 

 YOUNG f, and others, after coagulation by heat or alcohol, 

 is considered as muscular. But as it contains no fibrin, 

 and is even soluble in water, with the exception of a small 

 portion of extremely pellucid membrane, its muscular power 

 is denied by some. Its increasing density towards its cen- 

 tre, rather indicates a cellular structure, the cells being fil- 

 led with pellucid matter of different degrees of concentra- 

 tion. It may be added, as a still more decisive proof that 

 this power of varying the focal distance is not seated in the 

 crystalline, that when the lens is extracted in the disease 

 termed the Crystalline Cataract, the limits of distinct vision 

 suffer no diminution. 



When we look at objects within the limits of distinct vi- 

 sion, the iris expands, so that the aperture of the pupil be- 

 comes contracted. In this manner the least diverging rays 

 only are admitted, and distinct vision obtained. 



By this arrangement, it is probable that the eye accom- 

 modates itself to objects at different distances, within the 

 limits of distinct vision, as it is known to do by the same 

 means in reference to the quantity of light. 



The evidence in favour of this function of the iris ap- 

 pears to me to be conclusive. The enlargement and con- 

 traction of the pupil, are confined, indeed, within narrow 

 limits, but so is the extent of distinct vision within the or- 

 dinary limits. There is a particular range of minute in- 



* Oper. Om. p. T3. f Phil. Trans. 1793, p. 1C 9. 



