di Professor A. Thomson on the Phenomena 



recent observations by himself and others, tending to estabUsh a correct 

 view of the mechanism of this internal change of the organ of vision. 

 In the first part of the paper the structural arrangements within the 

 eye related to the process of adjustment were described. The author 

 referred — 1. To the structure and attachments of the cii'cular and 

 radiating fibres of the iris and of the ciliary muscle, pointing out the 

 cii'cumstances which prove incontestibly the muscular nature of these 

 fibres, and the manner in which theii* combined contraction has the 

 effect of compressing the lateral and posterior parts of the crystalline 

 lens, while they leave the anterior surface comparatively free. 2. The 

 author showed that the crystalline lens is suspended in such a manner 

 in its capsule, by means of elastic fibres, which pass both to its anterior 

 and posterior surfaces from the zonule of Zinn, that during the contrac- 

 tion of the ciliary muscle, these bands being shortened, the lens is per- 

 mitted to increase in thickness from before backwards ; but when the 

 ciliary muscle is relaxed, the elastic bands being drawn outwards, tend 

 to restore the flatter shape of the lens adapted for distant vision. 3. 

 The author showed that the space called posterior division of the 

 aqueous chamber of the eye, hitherto usually regarded as existing 

 between the iris and the lens, has no real existence ; for by sections of 

 the eye made in the frozen condition, it may be proved that the posterior 

 surface of the iris in its whole extent, excepting for a narrow space 

 close to the margin of the lens, Hes in accurate contact with the ante- 

 rior surface of the lens. The author exhibited to the Society a number 

 of sections of the human eye, made in the manner referred to. 



In the second part of the paper the author proceeded to consider the 

 phenomena which may be observed during changes of adjustment. He 

 referred more particularly to the recent catoptric observations of Cramer, 

 Helmholtz, and Bonders, from which it has been ascertained, that of the 

 dioptric media of the eye the crystalline lens is the only one which 

 undergoes a change, and that the change in the lens which accompanies 

 adjustment for near vision, consists mainly in the increased curvature 

 and advancement of its anterior surface ; while for distant vision this 

 part returns to its flatter condition. 



The author described a series of observations and experiments, from 

 which he had obtained the full confirmation of the results of the con- 

 tinental physiologists. These experiments consist mainly in the careful 

 observation of the reflected images of a candle or other light thrown 

 from the cornea, and from the anterior and posterior surfaces of the 

 lens, during efforts of adjustment. Of these images the corneal one is 

 found to undergo no perceptible change ; that from the posterior surface 

 of the lens becomes slightly smaller, but scarcely changes its position ; 



