OPTICS. Q55 



and are reciprocally as the distances of the same 

 object viewed in different places. 



The eye is in reality no more than a camera ob- 

 scura; for the rays of light flowing from all the 

 points of an object, through the pupil of the eye, 

 do by the refraction of its humours, paint the 

 image of it in the bottom of the eye: just so it is 

 in the camera obscura, where all the rays refracted 

 by a lens in the window-shutter, or passing through 

 a small hole in it, paint the image on the opposite 

 wall. 



The eye can only see a very small part of an 

 object distinctly at once. For the collateral parts 

 of an object are not represented distinctly in the 

 eye, and therefore the eye is forced to turn itself 

 successively to the several parts of the object it 

 wants to view, that they may fall near the axis of 

 the eye, where alone distinct vision is performed. 



When any point of an object is seen distinctly 

 with both eyes, the axes of both eyes are directed 

 to that point, and meet there; and then the object 

 appears single, though looked at with both eyes; 

 for the optic nerves are so framed, that the cor- 

 respondent parts in both eyes lead to the same 

 place in the brain, and give but one sensation ; and 

 the image will be twice as bright with both eyes as 

 with one. But if the axes of both eyes be not di- 

 rected to the object, that object Mdll appear double, 

 as the pictures in the two eyes do not fall upon 

 correspondent or similar parts of the retina. 



The best eye can hardly distinguish any objeet 

 that subtends at the eye an angle less than half a 

 minute; and very few can distinguish it when it 

 subtends a minute. 



Though men may see distinctly at different dis- 

 tances, by altering the position and figure of the 



