CHAP, in.] SIGHT. 11 



of diverging rays, which by the system are made to converge 

 again into the point in the image which corresponds to the point 

 in the object. One such pencil of rays proceeds from the point 

 at the extreme tip of the arrow, another from the extreme point 

 at the other end, and other pencils from all the points between 



FIG. 137. DIAGRAM OF THE FORMATION OF A RETINAL IMAGE. 



these two. Each such pencil has for its core a ray called the 

 principal ray, a, a', around which are arranged, with increasing 

 divergency, the other rays of the pencil, such as b, b', c, c' . When 

 such a pencil of rays falls on the refracting surface, such as the 

 'principal surface' of the reduced eye, the principal ray of the 

 pencil, a, being normal to that surface, is not refracted at all, but 

 passes straight on through the nodal point n, while the other rays 

 of the pencil, b, c, undergoing refraction according to their respec- 

 tive divergencies, are made to converge together at some point on 

 the path of the principal ray, and thus form at that spot the image 

 of the point from which the pencil proceeded. The exact position 

 on the line of the principal ray, at which convergence takes place 

 and at which the image is formed, will depend on the refractive 

 power of the optical system in relation to the amount of divergence 

 of the pencil; the refractive power of the system remaining the 

 same, it will be nearer to, or farther from, the nodal point according 

 as the rays are less or more divergent ; and the divergence of the 

 rays remaining the same, it will be nearer to, or farther from, the 

 nodal point according as the refractive power of the system is 

 greater or less. 



Hence supposing the eye to be in that condition in which a 

 distinct image of the arrow is formed on the retina, we can find 

 the position on the retina of the image of the extreme point of 

 the tip of the arrow, by simply drawing a .straight line from that 

 extreme point of the arrow X through the nodal point n of the 

 ' reduced ' eye. Such a straight line represents the path of the 

 ' principal ray ' of the pencil proceeding from the extreme tip 

 of the arrow, and when an image is formed on the retina the 

 other diverging rays of that pencil will be so refracted as to 

 converge at the point x, where that line meets the retina ; all 

 the rays will form together there the image of the extreme point 



