4 MICROSCOPE AND ACCESSORIES \CH. I 



more or less oblique to the principal axis. In Fig. 14, line (2), is a secondary 

 axis, and in Fig. 15, line (1). See also Fig. 65. 



\ 7. Principal Focus. — This is the point where rays parallel with the 

 axis and traversing^the lens'cross the axis ; and the distance from the focus to 

 the center of the lens measured along the axis is the Principal Focal Distance. 

 In the diagrams, Fig. 10 is seen to be a diverging lens, and the rays cross the 

 axis only by being projected backward. Such a focus is said to be virtual, as 

 it ha9 no real existence. In Fig. 11 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 sec- 

 ondary axes also, and each focus on a secondary axis has its conjugate. In 

 the formation of images the image is the conjugate of the object and con- 

 versely 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, 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 tjie 

 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 andf". 



<J 8. 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). 



\ 9. 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 thelight 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 (o i) , nearer the edge, 

 would come to a focus nearer the lens, the focus of the ray nearest the edge 

 being nearest the lens. 



