SPHERICAL ABERRATION. 



17 



may also be increased by combining two or three lenses into a 

 "system." 



There is a third position which the object may occupy, 

 namely, the principal focus. In this case no image at all is 

 formed, because rays diverging from any point of the object 

 emerge from the lens parallel to one another, and, therefore, 

 have no focus or image. The effects produced by a lens for 

 the three positions described may readily be verified by slowly 

 moving a lens, lying on a printed page, vertically upward, when 

 the letters will first appear erect, then vanish and finally return 

 in an inverted position. 



(2) By double concave lens. No real image is ever formed 

 by a concave lens, because it never forms a real focus; it is 

 always virtual, erect, smaller than the object and nearer the 



Fig. 16. 



lens than the object. If A be an object in Fig. 16, the rays of 

 light coming from any point of it will emerge from the lens 

 more divergent than before, and will appear to the eye to come 

 from a point nearer to the lens and to the axis as shown. 



Concave lenses are not used in the microscope except in com- 

 bination with convex lenses in some "objectives." 



SPHERICAL AND CHROMATIC ABERRATION. These are two seri- 

 ous inherent defects in all simple lenses. They are detrimental 

 to the formation of a perfect image of an object and must be 

 approximately overcome in compound microscopes if these are 

 to be of any value at all. 



CAUSE OF SPHERICAL ABERRATION. It was said above that 

 parallel rays of light falling upon a double convex lens are con- 

 verged to a point, but this is not quite true. The rays falling 

 on the edge of the lens are brought to a focus nearer to the lens 

 than those rays falling near the center of the lens, so that the 

 rays, instead of coming together in a point, are focused over a 

 small circle. The diagram (Fig. 17) will illustrate. 



