Appendix A: The Microscope and Its Optical Principles 135 



In the case of a concave lens, parallel rays will be caused to 

 diverge (Fig. 45) and the principal focus, F, of the lens is deter- 

 mined by the extension of the divergent rays till they meet at a 

 point which lies on the same side of the lens as the source of 

 light. Such a point has no actual existence, and is known, con- 

 sequently, as a virtual focus. The focus of a convex lens, on the 

 other hand, is real, and may be determined readily by allowing 

 the sun's rays, which are prac- 

 tically parallel, to pass through 

 it on to a screen. By moving 

 the lens backward and forward, 

 the spot of projected light varies 

 in size and brightness. When 

 smallest and brightest the spot 

 is at the focal point of the lens. 

 Images. In Fig. 46 the ob- 

 ject, represented by an arrow, 

 lies beyond the principal focus 

 of a convex lens as in a photo- 

 graphic camera, for example, or the objective of a compound 

 microscope. Light rays pass out in all directions from any 

 luminous point. Hence, one ray from any point on the arrow, 



the tip, for instance, 

 will pass through the 

 focal point, F, arid 

 one will pass through 

 the optical center of 

 FIG. 46. the lens. From what 



was determined above, manifestly the ray through F will emerge 

 as one of the parallel rays upon leaving the lens, and the one 

 through the optical center of the lens, since it traverses a 

 secondary axis, will not be refracted, hence the two rays must 

 cross. Their point of intersection is the point at which the 

 image of the arrow-tip will be formed. The same fact may be 

 determined, likewise, for any other point of the arrow, for 

 example, the opposite end. Thus the distance from the lens 



FIG. 45. 



