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remote point on the one line with the near point on the other cross 

 each other. Until the truth of this proposition shall have been de- 

 monstrated we are not at liberty to define a flat surface. This de- 

 monstration can only be obtained by experiment, and, therefore, it 

 was concluded, all our knowledge of geometry being founded on this 

 proposition, is experimental. 



2. Notice respecting recent Discoveries on the Adjustment 

 of the Eye to Distinct Vision. By Professor Goodsir. 



The question as to the arrangement by means of which the eye is 

 adapted for distinct vision at different distances has for two centuries 

 strongly attracted the attention of physiologists. The numerous 

 hypotheses, and untenable theories which have been advanced on 

 this subject are all, however, more or less unsatisfactory. They are 

 severally based on 



1. The mere structure or form of the refractive humours of the 

 eye; 



2. A presumed process connected with change in the direction of 

 the axis of vision ; 



3. The movements of the iris ; 



4. Change in the position of the retina ; 



5. Change in the position of the lens ; 



6. Change of form of the cornea ; 



7. Change of form of the lens. 



This important question has now been definitively determined by 

 the researches of Dr Cramer of Groningen, detailed in a prize treatise 

 submitted to the Dutch Association for the advancement of medical 

 science in 1851 ; but, which, except in the form of a short abstract 

 at the time, was only published at a later period. In 1853, Helm- 

 holtz also announced to the Berlin Academy the same discovery, 

 reached independently, and by a method more complex than that 

 employed by Cramer. 



The entire question had been previously simplified by the conclusion 

 to which Volkmann had come, that the eye, when in a passive con- 

 dition, is adapted for the vision of distant objects, the foci of con- 

 vergent pencils being then situated in the retina ; that when it re- 

 quires to be adjusted for a near object, an active pi'ocess of accom- 

 modation is set up, which brings the foci forward to the nervous 



