. The microscope; an introduction to microscopic methods and to histology. Microscopes. 292 CORRECTION OF APOCHROMATIC OBJECTIVES LCh. 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 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 comb
. The microscope; an introduction to microscopic methods and to histology. Microscopes. 292 CORRECTION OF APOCHROMATIC OBJECTIVES LCh. 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 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 i860 our own Charles A. Spencer used fluorspar in one of the combinations of his objectives (Proc. Acad. Nat. Sci., Phila., Vol. LVI (1904), p. 475; Trans. Amer. Micr. Soc, 1901, p. 23) § 466. Compensation oculars. — As the front lens of objectives of high power (fig. 21, 5 C) is not a combination but a single lens,. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Gage, Simon Henry, 1851-1944. [Ithaca, N. Y.
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