ch. r 



MICROSCOPE AND ACCESSORIES 



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. 



| 9. 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 dispersive power 

 are combined, concave lenses neutralizing the defects of convex lenses. If the 

 concave lens is not sufficiently strong to neutralize the aberrations of the convex 

 lens, the combination is said to be under-corrected, while if it is too strong and 

 brings the marginal raj's 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 contention 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 experiment, 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 \ 22 ) 



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

 preparation of microscopical objectives, oculars and illuminators. 



Figs. 14 and 15. 14. Convex lens 

 showing the position of the object (A-B ) 

 outside the principal focus {F i, and 

 the course of the rays in the formation 

 of real images. To avoid confusion the 

 rays are drawn from only one point. 



A B. Object outside the principal 

 focus. B' A'. Real, enlarged image 

 on the opposite side of the lens. 



Axis. Principal optic axis. 1,2,3. 

 Rays after traversing the lens. They 

 are converging, and consequently form 

 a real image. The dotted line and the 

 line 1 2 ) give the direction of the rays as 

 if unaffected by the lens. (F). The 

 principal focus. 



Fig. 15. — Convex lens, showing the 

 position of the object (A B) within the 

 principal focus and the course of rays 

 in the formation of a virtual image. 



A B. The object placed between the lens and its focus ; A 

 formed by tracing the rays backward. 

 the object, and is erect (\ 11). 



Axis. The principal optic axis of the lens. F. The principal focus. 



/,2,j. Rays from the point B of the object. They are diverging after trav- 

 ersing the lens, but not so divergent as if no lens were present, as is shown by the 



14 



15 



B' virtual image 

 It appears on the same side of the lens as 



