CH. I] MICROSCOPE AND ACCESSORIES 5 



\ 10. Correction of Chromatic and of Spherical Aberration. — Every 

 simple lens has the defect of both chromatic and spherical aberration, and to 

 overcome this, kinds of glass of different refractive power and different dis- 

 persive power are combined, concave lenses neutralizing the defects of convex 

 lenses. If the concave lens is not sufficiently strong to neutralize the aberra- 



Fig. 13. The ray (o) near 

 the edge of the lens is 

 brought to a focus nearer 

 the lens than the rav (i). 

 Both are brought to a focus 

 sooner than 'rays very near 

 the axis. (/) Principal 

 focus for rays very near the 

 axis ; (/')■ Focus for the ray 

 (i), and (f") Focus for the 



ray (0). Intermediate rays Fig. 13. Double Convex Lens, showing 

 would cross the axis all the Spherical Aberration, 



way from (f tof). 



tions of the convex lens, the combination is said to be under-corrected, while 

 if it is too strong and brings the marginal rays or the blue rays to a focus 

 beyond the true principal focus, the combination is over-corrected. 



In Newton's time there was supposed to be a direct proportion between 

 the refractive power of any transparent medium and its dispersive power (i. e. 

 its power to separate the light into colors). If this were true then the con- 

 tention of Newton that it would' be impossible to do away with the color 

 without at the same time doing away with the refraction would be true and 

 useful achromatic combinations would be impossible. It was found by experi- 

 ment, however, that there is not a direct ratio between the refractive and 

 dispersive powers for the different colors in different forms of glass, so that it 

 is possible to do away largely with chromatic aberration and retain sufficient 

 refraction to make the combination serve for the production of images. (See 

 also the discussion under apochromatic objectives \ 25.) 



Probably no higher technical skill is used in any art than is requisite in 

 the preparation of microscopical objectives, oculars and illuminators.* 



'<t 11. Geometrical Construction of Images. — As shown in Figs. 14-15, 

 for the determination of any point of an image, or the image being known, to 

 determine the corresponding part of the object, it is necessary to know the 

 position of the principal focus (and there is one on each side of the lens, .\ 7), 

 and the optical center of the lens (Figs. 1-9 ). Then a secondary axis (2) in 

 Fig. 14, (1) in Fig. 15, is drawn from the extremity of the object and pro- 

 longed indefinitely above the lens, or below it for virtual images. A second 

 line is drawn from the extremity of the object, (3) in Fig. 14, (2) in Fig. 15, 

 to the lens parallel with the principal, axis. After traversing the lens it must 

 he drawn through the principal focal point. If now it is prolonged it will 

 cross the secondary axis above the lens for a real image and below for a virtual 



