PHYSIOLOGICAL OPTICS 801 



properly centered prisms, converges the previously divergent rays, so that lumi- 

 nous point A is brought to a precise focus in C upon the principal axis. 



In ascertaining the formation of an image by a double convex lens, it must be 

 remembered that all objects possess numerous luminous points, the rays emitted by 

 them being collected by the lens into a corresponding number of foci. This implies 

 that under ordinary conditions, the image furnished by a biconvex lens, is real. 

 Supposing that we are dealing with a biconvex lens of the refractive power of the 

 lens of our eye, and place an object in front of it at a distance of more than twenty 

 feet, then the object emits, among others, a large number of rays which pursue a 

 course parallel to the principal axis of this lens (AB etc., Fig. 415). All these rays 

 are converged to very nearly the same focus F upon the principal axis. The dis- 



FIG. 415. DIAGRAM ILLUSTRATING THE REFRACTION OF PARALLEL RAYS BY A BICONVEX 



LENS. 



tance LF, is known as the principal focal distance. If the object is now moved 

 farther away from the lens, the principal focus F moves toward the lens, while if 

 the object is placed nearer the lens but not close enough to render the rays diver- 

 gent, the focal point F moves farther backward. Lastly, if F itself is rendered 

 luminous, the rays emitted from here traverse the lens in the opposite direction 

 and leave its anterior surface parallel to the principal axis. This is merely a re- 

 versal of the previous condition in which parallel rays are brought to a focus in F. 

 If a luminous point L is placed upon the principal axis at a distance greater 

 than the focal distance of this lens, but not far enough from it to cause its rays to 

 become parallel, then the rays diverging from it are brought to focus in L 1 , at a 



FIG. 416. DIAGRAM ILLUSTRATING THE REFRACTION OF DIVERGENT RAYS BY A BICONVEX 



LENS. 



point beyond the principal focus F (Fig. 416). In case L 1 is now rendered lumi- 

 nous, its rays are brought to a focus in L. For this reason, these points are com- 

 monly spoken of as conjugate foci. By moving luminous point L nearer to and 

 farther away from the lens, the focal point L 1 may be made to move first farther 

 away and then nearer to the lens. In the first case, a point will be reached when 

 the emerging rays finally become so greatly divergent that they cannot be focalized 

 at all (Fig. 417). This effect appears whenever the luminous point L is situated 

 nearer the lens than its principal focal distance. In this case, a virtual focus is 

 formed at L 1 , at the intersection of the prplongations of the emerging rays. 



If rays are directed into this lens which are already convergent, their conver- 

 gence is simply increased so that their focal point comes to lie nearer the lens than 



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