. Elements of applied microscopy. A text-book for beginners. Microscopy. 32 ELEMENTS OF APPLIED MICROSCOPY. bation which must be allowed for. Reference to Fig. 21 will show in a general way how a cover-glass shifts the rays coming from a point, F, and makes them arise apparently from F' and F". In the making of achro- matic objectives this effect of the cover-glass is taken into account; but with each lens, standard results can only be obtained with cover-glasses of a certain thickness. A variation of .05 mm. in thickness may quite obliterate. Fig. 21.—^DiSTUEBiNG Effect of the Cover-glas
. Elements of applied microscopy. A text-book for beginners. Microscopy. 32 ELEMENTS OF APPLIED MICROSCOPY. bation which must be allowed for. Reference to Fig. 21 will show in a general way how a cover-glass shifts the rays coming from a point, F, and makes them arise apparently from F' and F". In the making of achro- matic objectives this effect of the cover-glass is taken into account; but with each lens, standard results can only be obtained with cover-glasses of a certain thickness. A variation of .05 mm. in thickness may quite obliterate. Fig. 21.—^DiSTUEBiNG Effect of the Cover-glass. (After Gage.) certain fine structures. Adjustable objectives are so made that by turning a ring or collar the distance between their systems of lenses may be varied, being increased for the thinner cover-glasses. With an ordinary unad- justable objective one may select cover-glasses of the thickness for which it is corrected, the dimension being easily measured by some such apparatus, as is shown in Fig. 22. Or the lens system may be adjusted for cover- glasses other than the standard by altering the position of the draw-tube, changes in the tube-length producing changes similar to those which are provided for in the ad- justable objective. The tube-length should be increased for thin covers and decreased for those thicker than the. 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 Winslow, C. -E. A. (Charles-Edward Amory), 1877-1957. New York, J. Wiley; London, Chapman & Hall
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