844 FORMATION OF EETINAL IMAGES. [BOOK in. 



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 6, >', c, c r . When such a 

 pencil of rays falls on the refracting surface, such as the ' prin- 

 cipal 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 w, while the other rays of the 

 pencil, 5, c, undergoing refraction according to their respective 



FIG. 142. DIAGRAM OF THE FORMATION OF A RETINAL IMAGE. 



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 posi- 

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

 place and at which the image is formed, will depend on the re- 

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

 divergence of the pencil ; the refractive power of the system re- 

 maining 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 4 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 

 of the arrow. In a similar way a straight line drawn through 

 the nodal point from the extreme point of the other end of the 

 arrow, and continued until it meets the retina at y, will give us 

 the position of the image of the other end of the arrow ; and in 

 like manner lines drawn from other points of the arrow through 



