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mai-gin of the iris in the pupil, the erect image from the front of 

 the lens will be observed deeper and less distinct, a little beyond the 

 centre of the pupil, and the small distinct inverted image from the 

 back of the lens will be close to the opposite margin of the iris. 

 The eye being now adjusted to a near object, the deep erect image ad- 

 vances, diminishes, becomes more distinct, and moves across the centre 

 of the pupil to the immediate neighbourhood of the corneal image. 



This change in the relative position of the three images was cor- 

 rectly considered by Cramer as a distinct evidence of an increase in 

 the curvature of the anterior surface of the lens. It would appear, 

 however, that he was not entitled to conclude, as he did, from the 

 immobility of the inverted image, that no change occurs in the pos- 

 terior curvature of the lens. Donders, in reference to this has as- 

 serted, that the immobility of the inverted image affords satisfactory 

 evidence that a change does actually occur in the curvature of the 

 posterior surface of the lens ; and Stellwag has demonstrated that a 

 change of this kind must necessarily take place. That there is a 

 contemporaneous increase in the curvature of both surfaces of the 

 lens must be admitted from the consideration that if such a change 

 did not occur in the postei'ior surface, the increased curvature of the 

 anterior would necessarily produce a change in the position of the 

 inverted images ; which is not the case. The optical effect of the 

 increase of anterior curvature masks the slight movement of the in- 

 verted image. 



The alteration in the curvature of the posterior surface is, how- 

 ever, so slight, that we may safely assume that the essential altera- 

 tion takes place in the anterior surface. 



Helmholtz has proved that the anterior curvature of the lens is 

 increased during adjustment of the eye to near objects, by measur- 

 ing accurately the distance between the images of the flames of two 

 candles reflected from that surface, in the active and passive condi- 

 tions of accommodation. According to his calculations the radius of 

 curvature of the anterior surface is, for distant vision, from 10 to 11 

 millimetres ; for near vision about 5 millimetres. 



A change in the form of the lens having thus been ascertained to 

 be the mode of adjustment of the eye to distances ; the next point 

 to be determined is the mechanism by which the change of form is 

 effected. 



It may be stated generally, that although the structures which act 



