29 2 CORRECTION OF APOCHROMATIC OBJECTIVES LCn. IX 



visual and actinic foci are coincident, and if the apparatus is well 

 constructed there is never any difficulty in getting sharp pictures, for 

 the photographic image is sharpest when it appears sharpest to the 

 normal eye. 



FIG. 174. POSITIVE COMPENSATION OCULAR. 

 (From Spitta, p. no). 



C F C The field-lens is composed of two double convex crown lenses and one 

 double concave flint glass lens. 



C The eye-lens is of crown glass, and is separated from the field combination 

 the right distance to give the necessary excess magnification of the red image to 

 make it balance the blue image which was over magnified by the objective. 



Red Blue The red and blue rays limiting the image. It is seen here that the 

 rays are not parallel but divergent, as they extend above the ocular. When pro- 

 jected by the eye to the virtual image the rays cross, throwing the red one to the 

 outside, thus giving a larger image than is given by the blue ray, and the orange 

 haze at the margin of the field when looking through the ocular toward the window 

 or the sky. 



465a. It is interesting to note that the wonderful optical qualities of fluor- 

 spar were known to Sir David Brewster, and recommended by him for aid in achro- 

 matization (Brewster's work on the microscope, 1837, p. in); and before 1860 our 

 own Charles A. Spencer used fluorspar in one of the combinations of his objectives 

 (Proc. Acad. Nat. Sci., Phila., Vol. LVI (1904)^.475; Trans. Amer. Micr. Soc., 

 1901, p. 23) 



466. Compensation oculars. As the front lens of objectives of 

 high power (fig. 21, B C) is not a combination but a single lens, 



