848 



THE SENSE OF SIGHT 



reduced system is 50.8 diopters, and hence, the focal point of this 

 eye, when accommodated for a far object and in the position of rest, 

 lies precisely upon the retina. 



The Formation of the Retinal Image. In reducing the eye into 

 this simple form, Listing has followed the mathematical expositions 

 of Gauss 1 which show that the several media of any refractive system, 

 whenever centered upon the same optical axis, may be considered as 

 forming two parallel planes possessing an equal refractive power 

 (Fig. 448). For practical purposes, these two planes (P) with their 

 respective nodal points (N) may be regarded as being coincident, be- 

 cause the distance between them is actually very small so that the 

 refracted ray from the first plane is sent into the second still parallel to 

 the optical axis. In constructing the image of object AB, it must 

 be remembered that any luminous point upon AB sends out two rays, 

 one of which passes through the nodal point unrefracted, while the 



B 



FIG. 449. DIAGRAM TO ILLUS- 

 TRATE THE CONSTRUCTION NECES- 

 SARY TO DETERMINE THE LOCATION 

 AND SIZE OF THE RETINAL IMAGE. 



FIG. 448. DIAGRAM TO SHOW THE INVER- 

 SION OF THE IMAGE BY PARALLEL REFRACTING 

 SURFACES. 



AB, object; A l B l , image; N, nodal point 

 of two parallel refracting surfaces P; F, focal 

 point. 



other pursues a course parallel to the optical axis of this system and is 

 then refracted through its focal point F. At the point of intersection 

 of these two rays (A *) lies the image of luminous point A . If this con- 

 struction is repeated for luminous point B, it will be seen that the image 

 of AB is inverted. 



The same construction may be followed in the reduced eye (Fig. 

 449) , because we know the center of curvature (ri) of its single imaginary 

 refracting surface (R), in other words, its nodal point through which 

 all the principal rays may be imagined to enter the eye. These rays 

 are not deviated from their course, owing to the fact that they strike 

 the refracting surface at right angles. Consequently, all that is required 

 for the determination of the position of the image of an object upon the 

 retina, is to draw straight lines from its different luminous points 

 through the nodal point n It is evident that the retinal image is 

 inverted and that its size will be the smaller, the less the distance 

 of the nodal point from the retina and the greater its distance from the 

 object. Expressed in terms of the visual angle, it may then be said 



^ioptrische Untersuchungen, Gesellsch. der Wissensch., Gottingen, 1838-1843. 



