CH. /.] 



MICROSCOPE AND ACCESSORIES. 



and different dispersive power are combined, concave lenses neutraliz- 

 ing the defects of convex lenses. If the concave lens is not sufficiently 

 strong to neutralize the aberration of the convex lens, the combination 

 is said to be tinder-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. 



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

 in the preparation of microscopical objectives, oculars and illuminators. 



■>A r 



Figs. 14 and 15. 14. Convex lens 

 showing the position of the object [A-B) 

 outside the principal focus (E), 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 fo- 

 cus. 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 linage. The dotted line and the 

 line (2) give the direction of the rays 

 as if unaffected by the lens. (E). The 

 principal focus. 



Fig. 15. Convex lens showing the po- 

 sition 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' B' virtual image 

 formed by tracing the rays backward. It appears on the same side of the lens as 

 the object, and is erect ((! 9). 



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



1, 2, 3. Rays from the point B of the object. They are diverging after travers- 

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

 dotted lines. Ray (1) traverses the center of the lens, and is therefore not deviated. 

 It is a secondary axis (§5). 



AB 



14. 



IS- 



§ 8a. 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, \ 6), and the optical 

 center (Figs. 1-9) of the lens. Then a secondary axis, (2) in Fig. 14, (1) in Fig. 

 15, is drawn from the extremity of the object and prolonged 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 be drawn through the principal focal point. 



