MICROSCOPE AND ACCESSORIES 



[CH. I 



Fig. II the rays do cross the axis and the focus is said to be real. If the light 

 came from the opposite direction it would be seen that there is a principal focus 

 on the other side, that is there are two principal foci, one on each side of the lens. 

 These two foci are both principal foci, but they will be equally distant from the 

 center of the lens only when the curvature of the two lens surfaces are equal. 

 There may be foci on secondary axes also, each focus on a secondary axis has its 

 conjugate. In the formation of images the image is the conjugate of the object 

 and conversely the object is the conjugate of the image. 



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



The ray of white light (w) is represented as dividing into the short waved, bine 

 [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 ( /') 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 and f". 



'£ 7. Chromatic Aberration — This is due to the fact that ordinary light con- 

 sists of waves of varying length, and as the effect of a lens is to change the direc- 

 tion 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 (0) near the 

 edge of the lens is brought to a 



focus nearer the lens than the 

 ray (/). Both are brought to 

 a focus sooner than rays very 

 near the axis, (f) Principal 



focus for rays very near the 

 axis; (f) Focus for the ray 

 (*'), and [f") Focus for the ray 

 (o). Intermediate rays would 

 cross the axis all the "way from 

 if" tof). 



'i S. 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 



Fig. 



13. Double Convex Lens, showing 

 Spherical Aberration. 



