320 



THE SENSORIAL FUNCTIONS. 



each ray will, in that case, illuminate a different part of the 

 wall, and that the whole external scene will be there faith- 

 fully represented; for the several illuminated points, which 

 constitute these images, preserve among themselves the 

 same relative situation which the objects they represent do 

 in nature, although with reference to the actual objects they 

 have an inverted position. This inversion of the image is a 

 necessary consequence of the crossing of all the rays at the 

 same point; namely, the small aperture in the shutter, 

 through which they are admitted. 



One inconvenience attending the limiting of the illumina- 

 tion of each point of the wall to that of a single ray, in the 

 mode last pointed out, is that the image produced must ne- 

 cessarily be very faint. If, with a view of remedying this 

 defect, the aperture were enlarged, the image would, indeed, 

 become brighter, but would at the same time be rendered 

 more indistinct, from the intermixture and mutual interfe- 

 rence 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 ob- 

 tained with increased illumination, is to collect into one 

 point a great number of rays proceeding from the corre- 

 sponding point of the object to be represented. Such a col- 

 lection of rays proceeding from any point, is termed, in the 

 language of optics, a pencil of rays; and the point into 



