MICROSCOPE AND ACCESSORIES. 



\CH. I 



Fig. 12.— Double Convex Lens, Showing Chromatic Aberration. 



The ray of white light (a/) is represented as dividing into the short waved, blue 

 (b) and the long waved, red (r) light. The blue (b) ray comes to a focus nearer 

 the lens and the red ray {r) farther from the lens than the principal focus (/). 

 Principal focus {f) for rays very near the axis, f and f", foci of blue and red 

 light coming from near the edge of the lens. The intermediate wave lengths 

 would have foci all the way between f andf". 



\ 7. Chromatic Aberration. — This is due to the fact that ordinary light consists 

 of waves of varying length, and as the effect of a lens is to change the direction of 

 the waves, it changes the direction of the short waves more markedly than the 

 long waves. Therefore the short waved, blue light will cross the axis sooner than 

 the long waved, red light, and there will result a superposition of colored images, 

 none of which are perfectly distinct. (Fig. 12). 



Fig. 13. The ray (o) near the 

 edge of the lens is brought to a 

 focus nearer the lens than ike 

 ray(i). Both are brought to 

 a focus sooner than rays very 

 near the axis, if) Princi- 

 pal focus for rays very near 

 the axis ; (f) Focus for the 

 ray (i), and {f") Focus for 

 the ray (o). Intermediate 

 rays would cross the axis all 

 the way from [f" tof). 



Fig. 13. Double Convex Lens, showing 

 Spherical Aberration. 



§ 8. Spherical Aberration. — This is due to the unequal turning of 

 the light in different zones of a lens. The edge of the lens refracts 

 proportionally too much and hence the light will cross the axis or come 

 to a focus nearer the lens than a ray which is nearer the middle of the 

 lens. Thus, in Fig. 13, if the focus of parallel rays very near the axis 

 is at/, rays {0 z) nearer the edge would come to a focus nearer the 

 lens, the focus of the ray nearest the edge being nearest the lens. 

 Every simple lens has the defect of both chromatic and spherical aber- 

 ration, and to overcome this, kinds of glass of different refractive power 



