Abstract

The manipulations in using the microscope, other than the selection of lenses, focusing, and placement of slides, relate to the illumination. When the illumination is not handled with judgment, glare and lack of contrast result, and crisp images are not obtained. Effective illuminations depend equally upon three factors, the condenser, the field stop, and the illuminant proper. The adjustments for superior glare-free illumination are discussed and the term “controlled” illumination is suggested as an appropriate designation for such illumination. Aspects of the lighting requirements of the microscope are analyzed, and it is shown that suitable regulation of the apertures of the field and condenser stops are the basic requirements for controlled illumination. Various types of lighting may be used in conjunction with these adjustments. The paper is presented against a background of the pertinent historical developments and the various factors relating to illumination are reviewed in a broader fashion than is common in texts on microscopy. Explanations of the principles and the defects of illumination are presented with the general biological user of the microscope in mind. The shapes and proportions of the various illuminating beams that play on specimens are illustrated. Aberrations and the origin of glare are explained. In general, there are two extremes in the practical use of illumination—illumination for high resolution and illumination for maximum contrast. The microscopist should ordinarily vary his illumination as he works to obtain the advantages of each type of illumination. The conditions of each are fully treated.

© 1944 Optical Society of America

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References

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  1. John Belling, The Use of the Microscope (McGraw-Hill Book Company, Inc., New York, 1930). S. H. Gage, The Microscope (Comstock Publishing Company, Ithaca, New York, 1932). Conrad Beck, The Microscope, Theory and Practice (R. and J. Beck, London, 1938). F. J. Muñoz and H. A. Charipper, The Microscope and Its Use (Chemical Publishing Company, Brooklyn, New York, 1943). J. E. Barnard and F. V. Welch, Practical photomicrography (E. Arnold and Company, London, 1936). G. G. Reinert, Praktische Mikrofotografie (W. Knapp, Halle, 1937). R. M. Allen, Photomicrography (D. Van Nostrand Company, New York, 1941). C. P. Shillaber, Photomicrography in Theory and Practice (John Wiley & Sons, Inc., New York, 1944).
  2. I am greatly indebted to Dr. O. W. Richards, Spencer Lens Company, and Dr. G. L. Walls, Bausch and Lomb Optical Company, for their critical comments on a preliminary draft of the manuscript.
  3. Hugo von Mohl, Mikrographie, oder Anleitung zur Kenntniss und zum Gebrauche des Mikroskops (L. F. Fues, Tübingen, 1846). E. M. Nelson, J. Roy. Microscop. Soc. 7, 90–105 (1891).
  4. E. Abbe, Arch. f. mikroskop. Anat. 9, 469–480 (1873). Reprinted in Gesammelte Abhandhmgen von Ernst Abbe (G. Fischer, Jena, 1904), Vol. 1, and condensed translation in E. Abbe, Monthly Microscop. J. 13, 77–82 (1875).
    [Crossref]
  5. Nelson, reference 3. Focused illumination is but one of the possible techniques for securing effective illumination; it lends itself well to control. Control is the sine qua non of good illumination and, as will be shown, control is possible only when the plane of condenser focus is adjusted so that it falls at or near the level of objects (on a slide) in the field of view.
  6. Note in J. Roy. Microscop. Soc., p. 609 (1889).
  7. E. Abbe, Arch. f. mikroskop. Anat. 9, 413–468 (1873). Also in Gesammelte Abhandlungen von Ernst Abbe, Vol.  1, pp. 45–100.
    [Crossref]
  8. E. Abbe, J. Roy. Microscop. Soc., pp. 721–724 (1889).
    [Crossref]
  9. The distinction between wide and narrow cones is shown in Fig. 6.
  10. It should be stated emphatically at the outset that illumination intensity is a most important variable in microscopy; and where serious microscopy is attempted, a rheostat, neutral wedges, or filters should be employed to regulate intensity. The condenser is not for this purpose.
  11. E. M. Nelson, Eng. Mech. 40, 68, 157–158, 263, 282 (1884). E. M. Nelson, J. Roy. Microscop. Soc., pp. 282–289 (1910). Nelson, reference 3.
    [Crossref]
  12. Rayleigh, Phil. Mag. 42, [5], 167–195 (1896).
    [Crossref]
  13. L. C. Martin, An Introduction to Applied Optics (Isaac Pitman and Sons, London, Vol. 1, 1930; Vol. 2, 1932).
  14. N.A. is a numerical designation of the wideness of cone angle for the largest light cone that may be transmitted by a given objective. Similarly, it refers to the proportions of a beam emerging from a condenser and is correlated with the size of the condenser stop. N.A. = (the sine of the half-angle—in air—of the marginal rays of the light cone) × (the refractive index of the medium into which the cone is projected). Cf. Fig. 6E. The lower angle of the triangle is the half-angle of the light cone.
  15. For instance, in: C. R. Marshall and H. D. Griffith, An Introduction to the Theory and Use of the Microscope (G. Routledge and Sons, London, 1928), and Beck, reference 1. Shillaber, reference 1 shows photographs of starch grains with different illumination.
  16. The general method of examining the luminous appearance of an objective through the open draw tube (or by means of a suitable lens, through the ocular) was described by Abbe, reference 7.
  17. J. W. Gordon, J. Roy. Microscop. Soc., pp. 425–429 (1908). F. Welch, J. Roy. Microscop. Soc., pp. 34–37 (1930). R. E. Fitzpatrick, Stain Tech. 16, 107–109 (1941).
    [Crossref]
  18. M. H. Knisely, Anat. Rec. 64, 499–524 (1936).
    [Crossref]
  19. A. Köhler, Zeits. f. wiss. Mikrosk. 10, 433–440 (1893). A. Köhler, “Mikrophotographie,” Handb. der biol. Arbeitsmethoden (Abderhalden), Abt. 2, Part  2, No. 2, 1691–1978 (1931).
  20. Distributed for a time by the Zeiss Company. The more elaborate photomicrographic outfits are provided with lenses which can be combined to produce projecting lantern illumination.
  21. M. Berek, J. Roy. Microscop. Soc. 49, 240–244 (1929).
    [Crossref]
  22. Allen, reference 1.
  23. The pancratic condenser of the Zeiss Company (advertised about 1937 et seq.) for illumination of objectives between N.A. 0.16 and N.A. 1.40 should meet illumination requirements most satisfactorily.
  24. A. E. Wright, Principles of Microscopy, being a Handbook to the Microscope (The Macmillan Company, New York, 1917).
  25. A Spencer and a Leitz microscope, provided with apochromatic objectives, compensating type oculars, and corrected condensers, formed the basis for the various observations made. Occasionally, points were checked on student type microscopes of the same and other makers.
  26. The shape of the condenser beam is visualized in a dark room if a vertically held cigarette paper or a lightly exposed photographic film is moved back and forth through the beam. The proportions of the beam may be determined also by placing paper or a film horizontally on the microscope stage and racking the condenser up and down; the condenser beam is intercepted and a circle of measurable diameter may be noted for each level of the condenser. The circles are measured while they are being viewed by the microscope provided with lowest power lenses. An inch cube of uranium glass on the stage and in a darkened room shows a similar double cone from the condenser, but such a cone is a little less wide angled than one projected into air. Cf. Marshall and Griffith, reference 15; Muñoz and Charipper, reference 1. The cone within a slide in oil-immersion work is visualized this way.
  27. The effect of loss of contrast on visual resolution and the significance of other factors on visual performance in microscopy have been dealt with in a separate paper. J. Opt. Soc. Am. 34, 711 (1944).
    [Crossref]
  28. C. Beck, J. Roy. Microscop. Soc., pp. 399–405 (1922). C. Beck, J. Roy. Microscop. Soc., pp. 1–8 (1933). C. Fabry, Proc. Phys. Soc. London 48, 747–762 (1936).
    [Crossref]
  29. K. B. Blodgett, Phys. Rev. 55, 391–404 (1939). C. H. Cartwright, Phys. Rev. 57, 1060 (1940). F. L. Jones and H. J. Homer, J. Opt. Soc. Am. 31, 34–37 (1941).
    [Crossref]
  30. R. W. Wood, Physical Optics (The Macmillan Company, New York, 1934), third edition.
  31. The similarity of spherical aberration and glare in obscuring an image may be illustrated by looking through a microscope and condenser so adjusted that objects at a distance (i.e., out of a window) are seen as through a telescope. The degree of image fuzziness is largely owing to residual aberrations of the condenser. Opening the condenser increases spherical aberration and an increased haziness appears.
  32. Several slides, coated by the Cartwright method (reference 29), were prepared for me through the kindness of Professor R. C. Williams. When light of an appropriate color was used, a slight but perceptible decrease in glare was evident. Since coated slides do not alter the amount of internal reflection, the major problem remains.

1944 (1)

1939 (1)

K. B. Blodgett, Phys. Rev. 55, 391–404 (1939). C. H. Cartwright, Phys. Rev. 57, 1060 (1940). F. L. Jones and H. J. Homer, J. Opt. Soc. Am. 31, 34–37 (1941).
[Crossref]

1936 (1)

M. H. Knisely, Anat. Rec. 64, 499–524 (1936).
[Crossref]

1929 (1)

M. Berek, J. Roy. Microscop. Soc. 49, 240–244 (1929).
[Crossref]

1922 (1)

C. Beck, J. Roy. Microscop. Soc., pp. 399–405 (1922). C. Beck, J. Roy. Microscop. Soc., pp. 1–8 (1933). C. Fabry, Proc. Phys. Soc. London 48, 747–762 (1936).
[Crossref]

1908 (1)

J. W. Gordon, J. Roy. Microscop. Soc., pp. 425–429 (1908). F. Welch, J. Roy. Microscop. Soc., pp. 34–37 (1930). R. E. Fitzpatrick, Stain Tech. 16, 107–109 (1941).
[Crossref]

1896 (1)

Rayleigh, Phil. Mag. 42, [5], 167–195 (1896).
[Crossref]

1893 (1)

A. Köhler, Zeits. f. wiss. Mikrosk. 10, 433–440 (1893). A. Köhler, “Mikrophotographie,” Handb. der biol. Arbeitsmethoden (Abderhalden), Abt. 2, Part  2, No. 2, 1691–1978 (1931).

1889 (2)

Note in J. Roy. Microscop. Soc., p. 609 (1889).

E. Abbe, J. Roy. Microscop. Soc., pp. 721–724 (1889).
[Crossref]

1884 (1)

E. M. Nelson, Eng. Mech. 40, 68, 157–158, 263, 282 (1884). E. M. Nelson, J. Roy. Microscop. Soc., pp. 282–289 (1910). Nelson, reference 3.
[Crossref]

1873 (2)

E. Abbe, Arch. f. mikroskop. Anat. 9, 413–468 (1873). Also in Gesammelte Abhandlungen von Ernst Abbe, Vol.  1, pp. 45–100.
[Crossref]

E. Abbe, Arch. f. mikroskop. Anat. 9, 469–480 (1873). Reprinted in Gesammelte Abhandhmgen von Ernst Abbe (G. Fischer, Jena, 1904), Vol. 1, and condensed translation in E. Abbe, Monthly Microscop. J. 13, 77–82 (1875).
[Crossref]

Abbe, E.

E. Abbe, J. Roy. Microscop. Soc., pp. 721–724 (1889).
[Crossref]

E. Abbe, Arch. f. mikroskop. Anat. 9, 469–480 (1873). Reprinted in Gesammelte Abhandhmgen von Ernst Abbe (G. Fischer, Jena, 1904), Vol. 1, and condensed translation in E. Abbe, Monthly Microscop. J. 13, 77–82 (1875).
[Crossref]

E. Abbe, Arch. f. mikroskop. Anat. 9, 413–468 (1873). Also in Gesammelte Abhandlungen von Ernst Abbe, Vol.  1, pp. 45–100.
[Crossref]

Allen,

Allen, reference 1.

Beck, C.

C. Beck, J. Roy. Microscop. Soc., pp. 399–405 (1922). C. Beck, J. Roy. Microscop. Soc., pp. 1–8 (1933). C. Fabry, Proc. Phys. Soc. London 48, 747–762 (1936).
[Crossref]

Belling, John

John Belling, The Use of the Microscope (McGraw-Hill Book Company, Inc., New York, 1930). S. H. Gage, The Microscope (Comstock Publishing Company, Ithaca, New York, 1932). Conrad Beck, The Microscope, Theory and Practice (R. and J. Beck, London, 1938). F. J. Muñoz and H. A. Charipper, The Microscope and Its Use (Chemical Publishing Company, Brooklyn, New York, 1943). J. E. Barnard and F. V. Welch, Practical photomicrography (E. Arnold and Company, London, 1936). G. G. Reinert, Praktische Mikrofotografie (W. Knapp, Halle, 1937). R. M. Allen, Photomicrography (D. Van Nostrand Company, New York, 1941). C. P. Shillaber, Photomicrography in Theory and Practice (John Wiley & Sons, Inc., New York, 1944).

Berek, M.

M. Berek, J. Roy. Microscop. Soc. 49, 240–244 (1929).
[Crossref]

Blodgett, K. B.

K. B. Blodgett, Phys. Rev. 55, 391–404 (1939). C. H. Cartwright, Phys. Rev. 57, 1060 (1940). F. L. Jones and H. J. Homer, J. Opt. Soc. Am. 31, 34–37 (1941).
[Crossref]

Gordon, J. W.

J. W. Gordon, J. Roy. Microscop. Soc., pp. 425–429 (1908). F. Welch, J. Roy. Microscop. Soc., pp. 34–37 (1930). R. E. Fitzpatrick, Stain Tech. 16, 107–109 (1941).
[Crossref]

Griffith, H. D.

For instance, in: C. R. Marshall and H. D. Griffith, An Introduction to the Theory and Use of the Microscope (G. Routledge and Sons, London, 1928), and Beck, reference 1. Shillaber, reference 1 shows photographs of starch grains with different illumination.

Knisely, M. H.

M. H. Knisely, Anat. Rec. 64, 499–524 (1936).
[Crossref]

Köhler, A.

A. Köhler, Zeits. f. wiss. Mikrosk. 10, 433–440 (1893). A. Köhler, “Mikrophotographie,” Handb. der biol. Arbeitsmethoden (Abderhalden), Abt. 2, Part  2, No. 2, 1691–1978 (1931).

Marshall, C. R.

For instance, in: C. R. Marshall and H. D. Griffith, An Introduction to the Theory and Use of the Microscope (G. Routledge and Sons, London, 1928), and Beck, reference 1. Shillaber, reference 1 shows photographs of starch grains with different illumination.

Martin, L. C.

L. C. Martin, An Introduction to Applied Optics (Isaac Pitman and Sons, London, Vol. 1, 1930; Vol. 2, 1932).

Nelson, E. M.

E. M. Nelson, Eng. Mech. 40, 68, 157–158, 263, 282 (1884). E. M. Nelson, J. Roy. Microscop. Soc., pp. 282–289 (1910). Nelson, reference 3.
[Crossref]

Rayleigh,

Rayleigh, Phil. Mag. 42, [5], 167–195 (1896).
[Crossref]

von Mohl, Hugo

Hugo von Mohl, Mikrographie, oder Anleitung zur Kenntniss und zum Gebrauche des Mikroskops (L. F. Fues, Tübingen, 1846). E. M. Nelson, J. Roy. Microscop. Soc. 7, 90–105 (1891).

Wood, R. W.

R. W. Wood, Physical Optics (The Macmillan Company, New York, 1934), third edition.

Wright, A. E.

A. E. Wright, Principles of Microscopy, being a Handbook to the Microscope (The Macmillan Company, New York, 1917).

Anat. Rec. (1)

M. H. Knisely, Anat. Rec. 64, 499–524 (1936).
[Crossref]

Arch. f. mikroskop. Anat. (2)

E. Abbe, Arch. f. mikroskop. Anat. 9, 469–480 (1873). Reprinted in Gesammelte Abhandhmgen von Ernst Abbe (G. Fischer, Jena, 1904), Vol. 1, and condensed translation in E. Abbe, Monthly Microscop. J. 13, 77–82 (1875).
[Crossref]

E. Abbe, Arch. f. mikroskop. Anat. 9, 413–468 (1873). Also in Gesammelte Abhandlungen von Ernst Abbe, Vol.  1, pp. 45–100.
[Crossref]

Eng. Mech. (1)

E. M. Nelson, Eng. Mech. 40, 68, 157–158, 263, 282 (1884). E. M. Nelson, J. Roy. Microscop. Soc., pp. 282–289 (1910). Nelson, reference 3.
[Crossref]

J. Opt. Soc. Am. (1)

J. Roy. Microscop. Soc. (5)

C. Beck, J. Roy. Microscop. Soc., pp. 399–405 (1922). C. Beck, J. Roy. Microscop. Soc., pp. 1–8 (1933). C. Fabry, Proc. Phys. Soc. London 48, 747–762 (1936).
[Crossref]

M. Berek, J. Roy. Microscop. Soc. 49, 240–244 (1929).
[Crossref]

Note in J. Roy. Microscop. Soc., p. 609 (1889).

J. W. Gordon, J. Roy. Microscop. Soc., pp. 425–429 (1908). F. Welch, J. Roy. Microscop. Soc., pp. 34–37 (1930). R. E. Fitzpatrick, Stain Tech. 16, 107–109 (1941).
[Crossref]

E. Abbe, J. Roy. Microscop. Soc., pp. 721–724 (1889).
[Crossref]

Phil. Mag. (1)

Rayleigh, Phil. Mag. 42, [5], 167–195 (1896).
[Crossref]

Phys. Rev. (1)

K. B. Blodgett, Phys. Rev. 55, 391–404 (1939). C. H. Cartwright, Phys. Rev. 57, 1060 (1940). F. L. Jones and H. J. Homer, J. Opt. Soc. Am. 31, 34–37 (1941).
[Crossref]

Zeits. f. wiss. Mikrosk. (1)

A. Köhler, Zeits. f. wiss. Mikrosk. 10, 433–440 (1893). A. Köhler, “Mikrophotographie,” Handb. der biol. Arbeitsmethoden (Abderhalden), Abt. 2, Part  2, No. 2, 1691–1978 (1931).

Other (19)

Distributed for a time by the Zeiss Company. The more elaborate photomicrographic outfits are provided with lenses which can be combined to produce projecting lantern illumination.

L. C. Martin, An Introduction to Applied Optics (Isaac Pitman and Sons, London, Vol. 1, 1930; Vol. 2, 1932).

N.A. is a numerical designation of the wideness of cone angle for the largest light cone that may be transmitted by a given objective. Similarly, it refers to the proportions of a beam emerging from a condenser and is correlated with the size of the condenser stop. N.A. = (the sine of the half-angle—in air—of the marginal rays of the light cone) × (the refractive index of the medium into which the cone is projected). Cf. Fig. 6E. The lower angle of the triangle is the half-angle of the light cone.

For instance, in: C. R. Marshall and H. D. Griffith, An Introduction to the Theory and Use of the Microscope (G. Routledge and Sons, London, 1928), and Beck, reference 1. Shillaber, reference 1 shows photographs of starch grains with different illumination.

The general method of examining the luminous appearance of an objective through the open draw tube (or by means of a suitable lens, through the ocular) was described by Abbe, reference 7.

The distinction between wide and narrow cones is shown in Fig. 6.

It should be stated emphatically at the outset that illumination intensity is a most important variable in microscopy; and where serious microscopy is attempted, a rheostat, neutral wedges, or filters should be employed to regulate intensity. The condenser is not for this purpose.

Nelson, reference 3. Focused illumination is but one of the possible techniques for securing effective illumination; it lends itself well to control. Control is the sine qua non of good illumination and, as will be shown, control is possible only when the plane of condenser focus is adjusted so that it falls at or near the level of objects (on a slide) in the field of view.

John Belling, The Use of the Microscope (McGraw-Hill Book Company, Inc., New York, 1930). S. H. Gage, The Microscope (Comstock Publishing Company, Ithaca, New York, 1932). Conrad Beck, The Microscope, Theory and Practice (R. and J. Beck, London, 1938). F. J. Muñoz and H. A. Charipper, The Microscope and Its Use (Chemical Publishing Company, Brooklyn, New York, 1943). J. E. Barnard and F. V. Welch, Practical photomicrography (E. Arnold and Company, London, 1936). G. G. Reinert, Praktische Mikrofotografie (W. Knapp, Halle, 1937). R. M. Allen, Photomicrography (D. Van Nostrand Company, New York, 1941). C. P. Shillaber, Photomicrography in Theory and Practice (John Wiley & Sons, Inc., New York, 1944).

I am greatly indebted to Dr. O. W. Richards, Spencer Lens Company, and Dr. G. L. Walls, Bausch and Lomb Optical Company, for their critical comments on a preliminary draft of the manuscript.

Hugo von Mohl, Mikrographie, oder Anleitung zur Kenntniss und zum Gebrauche des Mikroskops (L. F. Fues, Tübingen, 1846). E. M. Nelson, J. Roy. Microscop. Soc. 7, 90–105 (1891).

R. W. Wood, Physical Optics (The Macmillan Company, New York, 1934), third edition.

The similarity of spherical aberration and glare in obscuring an image may be illustrated by looking through a microscope and condenser so adjusted that objects at a distance (i.e., out of a window) are seen as through a telescope. The degree of image fuzziness is largely owing to residual aberrations of the condenser. Opening the condenser increases spherical aberration and an increased haziness appears.

Several slides, coated by the Cartwright method (reference 29), were prepared for me through the kindness of Professor R. C. Williams. When light of an appropriate color was used, a slight but perceptible decrease in glare was evident. Since coated slides do not alter the amount of internal reflection, the major problem remains.

Allen, reference 1.

The pancratic condenser of the Zeiss Company (advertised about 1937 et seq.) for illumination of objectives between N.A. 0.16 and N.A. 1.40 should meet illumination requirements most satisfactorily.

A. E. Wright, Principles of Microscopy, being a Handbook to the Microscope (The Macmillan Company, New York, 1917).

A Spencer and a Leitz microscope, provided with apochromatic objectives, compensating type oculars, and corrected condensers, formed the basis for the various observations made. Occasionally, points were checked on student type microscopes of the same and other makers.

The shape of the condenser beam is visualized in a dark room if a vertically held cigarette paper or a lightly exposed photographic film is moved back and forth through the beam. The proportions of the beam may be determined also by placing paper or a film horizontally on the microscope stage and racking the condenser up and down; the condenser beam is intercepted and a circle of measurable diameter may be noted for each level of the condenser. The circles are measured while they are being viewed by the microscope provided with lowest power lenses. An inch cube of uranium glass on the stage and in a darkened room shows a similar double cone from the condenser, but such a cone is a little less wide angled than one projected into air. Cf. Marshall and Griffith, reference 15; Muñoz and Charipper, reference 1. The cone within a slide in oil-immersion work is visualized this way.

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Figures (9)

Fig. 1
Fig. 1

AG: Several cells of monkey parathyroid gland stained for mitochondrial granules in the cytoplasm; 3μ section ×1500. AD shows the appearance with oil-immersion (A); and with 4-mm (B), 8-mm (C), and 16-mm (D) objectives, each with full cone illumination. The numerator and denominator represent, respectively, the N.A. of the objective and that of the condenser. EG are oil-immersion figures but with reduced condenser apertures as indicated. H shows two globules of mercury photographed with reduced and with full cone illumination; the former illumination results in diffraction bands, ×1500.

Fig. 2
Fig. 2

Standard illuminating mechanisms. A, Nelson’s method for critical illumination. B, ground-glass diffusing screen, C, speculum. D, episcope illumination. E, magnified source illumination. F and G, projection lantern type illumination. In the compound systems, E, F, and G, the level of the source image is shown at F and F′; arrows indicate the levels of field stops on which the condenser and microscope are focused.

Fig. 3
Fig. 3

Aspects of image formation by the condenser. To the right (the condenser field) and to the left (the region of focus), lines A, B, and C and a, b, c represent the location of conjugate points, Aa, etc.

Fig. 4
Fig. 4

Diagram of the optical beam through the microscope and eye. Greek letters refer to segments of the beam; A, B, C, and D are levels of simultaneous focus; numbers refer to sites of glare production.

Fig. 5
Fig. 5

To satisfy completely microscope requirements for the various objective-ocular combinations, beams must have the proportions shown below and to the right.

Fig. 6
Fig. 6

The shapes of beams emerging from the condenser; A, with wide condenser and field stops; B, with wide open condenser stop and no field stop; C, with narrow condenser and field stops (x is the level of the image of the field stop and y that of the condenser stop); D, condenser and field stops regulated for controlled illumination of the 4-mm objective; E, same for the 8-mm objective (o = objective, c = condenser, z = level at which the field stop image appears; the lower angle of the triangle is the half-angle of the beam and represents the angular aperture of the condenser beam); F, controlled illumination for the 16-mm objective.

Fig. 7
Fig. 7

A shows the refraction of rays A, B, and C through the first lens of an objective; rays B and C outside the field of view of the objective (zone A) are reflected (R) on lens mountings as glare producing light. B shows two rays (A and B) reflected (internal reflection) within the front lens of an objective and forming a source of glare (R).

Fig. 8
Fig. 8

A, ray tracing from condenser (C) through slide (SL) and into objective (O). B, same but with oil-immersion (oil-stippled). C, arrows represent external and internal reflections at the several surfaces. D, spherical aberration in a beam emerging from an unimmersed condenser top lens. E, centrally directed reflections with a high condenser cone. F, peripherally directed reflections with a low condenser cone. G, objects on a slide below the level of focus (F) of a high condenser cone. H, increased contrast of particles caused by less top illumination when the level of focus (F) is below the objects.

Fig. 9
Fig. 9

Ray tracing through a lens showing spherical aberration. Focus level is over the span XY. B, rays converging to a focus at O are distorted by spherical aberration when a flat plate is interposed and the focus spreads between X and Y. C, rays diverging from a point (O) are distorted in traversing a plate so that the projections of the emerging rays intersect between X and Y. D, spherical aberration of a lens with a zone of focus at XY. Point S is projected to level R as a blur circle of the size shown at S′.