VISION. 405 



One inconvenience attending the limiting of the 

 illumination of each point of the wall to that of a 

 single ray, in the mode last pointed out, is that the 

 image produced must necessarily be very faint. 

 If, with a view of remedying this defect, the aper- 

 ture were enlarged, the image would, indeed, become 

 brighter, but would at the same time be rendered 

 more indistinct, from the intermixture and mutual 

 interference of adjacent rays ; for all the lines would 

 be spread out, the outlines shaded off, and the whole 

 picture confused. 



The only mode by which distinctness of image 

 can be obtained with increased illumination, is to 

 collect into one point a great number of rays pro- 

 ceeding from the corresponding point of the object 

 to be represented. Such a collection of rays pro- 

 ceeding from any point, is termed, in the language 

 of optics, 2i pencil of rays; and the point into which 

 they are collected is called a focus. For the pur- 

 pose of collecting a pencil of rays into a focus, it is 

 evident that all of them, except the one which 

 proceeds in a straight line from the object to that 

 focus, must be deflected, or bent from their rectili- 

 neal course. This effect may be produced by re- 

 fraction, which takes place according to another 

 optical law; a law of which the following is the 

 exposition. 



It is only when the medium which the rays are 

 traversing is of uniform density that their course is 

 constantly rectilineal. If the density change, or if 

 the rays pass obliquely from one medium into 

 another of a different density, they are refracted ; 

 each ray being deflected towards a line situated in 

 the medium of greatest density, and drawn from 



