Otjecls distlnclhj at different Distances. 343 



When an object is seen distinctly, its image is painted at 

 the same invariable distance from the crystaUine lens, 

 Avhether the object be near or remote. 



The same Propertij cf Vision demonstrated hj Experiments. 



It is a property of tlie convex lens, that when its aperture 

 is contracted, the image of a near object is painted at the 

 same distance from the glass, as a remote one when its 

 apertttre is enlarged. 



The instrument with which I made the following expe- 

 riments consists of two large tubes, one within the other, 

 like the tubes of a telescope. At the end of the large Uibe 

 a double convex lens of 24 inches focus is fixed to represent 

 the crystalline lens ; and the other end of the instrument 

 contains a piece of ground glass, 4| inches in diameter, 

 polished on one side, to represent the retina. This i-nstrument, 

 being fixed upon a proper stand, was carried into a room 

 and placed at about five feet from a window from which 

 I had an extensive view. 



First Experiment. 



After I had contracted the object glass to two inches, I 

 drew out the inner tube until the picture of remote objects 

 was distinctly painted upon the rough glass, but near ob- 

 jects were represented imperfectly.. 



Second Experiments 

 When T had contracted the aperture of the Jens to half 

 an inch, near objects were represented very distinctly, but 

 the picture of remote objects was less conspicuous. 



Third Experiment. 

 The aperturj of the lens being further contracted to \ of 

 an inch in diameter, all outward objects appeared indistinct ; 

 but a plumb-line, which hung down the middle of the win- 

 dow, was clearly represented upon the artificial retina. This 

 line was not seen upon the retina in the first exiieriment, 

 and only like an enlarged shadow in the second. The di- 

 stance between the glasses was the same in all the experi- 

 y 4 mcnts. 



