visiOxN. 423 



by a voluntary effort, of converting the double 

 image into a single one, or vice versa ; a change 

 which, being quite independent of the contraction 

 or dilatation of the pupil, and of any change ex- 

 ternal to the globe of the eye, can result only from 

 a change effected in the refracting powers of the 

 eye.* 



The mode in which this change in the state of 

 the eye is effected has been the subject of much 

 controversy. The increase of the refracting power 

 of the eye necessary to adapt it to the vision of 

 near objects is evidently the result of a muscular 

 effort, of which we are distinctly conscious when 

 we accurately attend to the accompanying sen- 

 sations. The researches of Dr. Young have ren- 

 dered it probable that some change takes place in 

 the figure of the lens, whereby its convexity, and 

 perhaps also its distance from the retina, are in- 

 creased. He has shown by a very decisive experi- 

 ment, that any change which may take place in the 

 convexity of the cornea has but little share in the 

 production of the effect; for the eye retains its 

 power of adaptation when immersed in water, in 

 which the form of the cornea can in no respect 

 influence the refraction. 



But the rays of light are of different kinds; some 

 exciting the sensation of red, others of yellow, and 

 others again of blue ; and these different species of 



* This decisive experiment was first pointed out by Dr. Porter- 

 field, in 1759. On the same principle Dr. Young contrived an 

 instrument, which he termed the Ojjfometer, (in which a line in a 

 - flat graduated ruler is viewed through two narrow vertical slits made 

 in a piece of brass), for measuring with exactness the distance to 

 which the refractive powers of the eye are adjusted. 



