and Causes of the Focal Adjustment of the Eye. 95 



but the dull, erect image, thrown from the anterior surface of the lens, 

 is observed at every change of adjustment from distant to near vision, 

 suddenly to approach the corneal image, and to become somewhat 

 smaller and more distinct. By the employment of a double light for 

 reflection, and an ingenious instrument, which he has named opthalmo- 

 raeter, Helmholtz has contrived to render the phenomena still more 

 apparent, and to measure with precision the amount of the change in 

 the radius of curvature of the anterior surface of the lens. The general 

 conclusion from aU the observations is, that during adjustment for near 

 vision the lens is diminished in its transverse, and increased in its 

 antero-posterior diameter ; that the bulging or increased curvature takes 

 place principally in the middle part, while the marginal part of the lens 

 is cai-ried somewhat backwards ; that the posterior surface of the lens 

 undergoes a slight increase of curvature, but does not change its place 

 at its central part ; and that the cornea undergoes no change whatever, 

 either in form or place. The whole changes by which adjustment is 

 effected belong, therefore, to the lens, and to those structures connected 

 with it, which contribute to the alteration of its form. 



In the third pai't of the paper the author endeavoured to explain the 

 mechanism by which the changes of form of the lens, occurring in adjust- 

 ment, are produced. This explanation, he showed, is to be sought for 

 in a knowledge of the structure and actions of the internal parts of the 

 eye, described in the first part of the paper. 



1. The mode of attachment of the capsule of the lens, by means of 

 the zonule of Zinn and ciliary pi-ocesses, to the choroid membrane and 

 external coats of the eye-ball, is such as to prevent the lens from reced- 

 ing to any appreciable extent in the vitreous humour, while the condi- 

 tion of the iris in front of the lens, especially at or near the pupil, offers 

 no resistance to its advancement and bulging in that direction. 



2. The effect of the simultaneous contraction of both sets of fibres 

 of the iris, and of the ciliary muscle, is to compress the lens laterally ; 

 and this pressure is rendered more equal and diffused by the structure 

 termed canal of Petit and the fluid which surrounds the margin of the 

 lens. The result of this lateral compression is the anterior bulging 

 of the lens and the change of its curvature previously referred to. 



The author showed that although the contraction of the circular 

 fibres of the iris (and consequent diminution of the size of the pupil) is 

 a very constant accompaniment of the adjustment for near vision, tliere 

 are yet reasons for believing that the ciliary muscle is the most direct 

 agent in producing the required change of form of the lens, and that 

 the iris is rather only an assistant agent in this action ; and in connec- 

 tion with this subject the author described several cases which had come 



