N O V E M B E R , 1 9 4 0 J . O . S . A . V O L U M E 3 0

A Simple Method for Determining Chromatic Aberration for Photomicrographic Purposes

WALTER KOCH Hebrew University, Jerusalem, Palestine

(Received September 21, 1940)

IN apochromatic object glasses the chromatic error within the visible range of the spectrum

is eliminated with an exactitude sufficient for most purposes. But achromatic lenses are, in most cases, sufficient in photomicrography, especially if the most suitable part of the spec­trum is selected by means of a green filter.

Some difficulties arise if invisible portions of the spectrum are to be used. Here the chromatic aberration is so considerable that pictures of inferior quality result and the difference in focus can only be found by a long series of test ex­posures.

Several methods have been advocated to limit the number of necessary test exposures. For photography in the infra-red, Zeiss recom­mends focusing through a red color filter at the limit of the visible red. The difference of focus to infra-red, and the resulting lack of sharpness, are thus smaller than after focusing, for example, in the green.

Another method is employed by Naumann.1

This author recommends focusing through a green filter (Agfa No. 70) and registering the position of the graduated drum of the microm­eter screw. Then the object is focused through a red filter (Agfa No. 42) and the position registered again. Twice the difference between these two readings gives approximately the right focus for 820 mµ.

A similar method is used by Trivelli. and Foster.2 The object is focused visually through a Wratten filter No. 62. Then an exposure is made in ultraviolet through filter No. 18A (365 mµ). The resulting picture is diffuse. If, now, a similar degree of diffuseness is produced through filter No. 62, an exposure through filter No. 18 will render an approximately sharp picture. Here, too, the final adjustment in the range in question is done by test exposures.

1 H. Naumann, Zeits. f. Instrumentenk. 54, 276 (1934). 2 A. P. H. Trivelli and L. V. Foster. "Photomicrography with the 365-mµ mercury arc line." J. Opt. Soc. Am. 21, 124 (1931).

Coincidence of visual focusing with ultra­violet is attained by Bausch & Lomb through specially manufactured object lenses in which the chromatic difference between 365 mµ and 546 mµ is eliminated. Thus, when focused at a wave-length of 546 mµ, an exposure at 365 mµ will be sharp. However, when high apertures are employed, the focuses of the two wave-lengths differ slightly, and, therefore, the final focusing has to be done by test exposures after all.

The method of Zeiss has the disadvantage that the difference of focus between red and infra­red is small only for portions of the spectrum which lie close to each other. There is a similar drawback in Naumann's method, for the differ­ences of focus are not directly proportional to the wave-lengths. Also, the method of Trivelli and Foster requires a considerable number of test exposures.

In the following, a method is described which allows one to find the difference of focus by a single test exposure.

PRINCIPLE OF THE METHOD

A stage micrometer is photographed in oblique position in the light of different wave­lengths and the difference of focus is read from the division marks of the stage micrometer.

The following instruments are required: a microscope with graduated drum and a stage micrometer. The determination will be easier and more precise when using a microscope with a mechanical stage and one or two wedges and an S-shaped piece of tin which may be manufac­tured by any workman. The measurements of these wedges need not be exact since the adjust­ment is done with the scale of the microscope.

PROCEDURE

(a) Microscope without mechanical stage (1) The stage micrometer is photographed

through a green filter. This exposure can be 564

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omitted if the flatness of field is such that when focusing visually the division marks of the stage micrometer appear sharp all over the field of vision.

(2) One end of the stage micrometer is then lifted about 1 cm (when powerful object glasses are used) or 2 cm (with low object glasses), thus bringing the division marks into different dis­tances from the objective lens. The difference of height between the highest and the lowest division mark is measured as follows: By means of the stage motions both ends are subsequently centered and focused. The respective values on the graduated drum are recorded and the value of each mark can easily be calculated. The exactitude of measurement can be increased by employing more powerful object lenses and eye­pieces.

(3) Now the middle of the stage micrometer is focused through a green or red filter and photo­graphed. The central division mark will appear sharp.

(4) Without altering the focus, the red or green color filter is replaced by a filter of the re­quired portion of the spectrum (e.g., infra-red) and another exposure made. Now, owing to the

chromatic difference, another division mark will appear sharp. This will indicate the new plane of focus.

(5) If the stage micrometer has m division marks and the difference of height is n mm, the value of one division mark is n/m mm. If the pth mark (counted from the middle) appears sharp in the picture, the necessary alteration of focus is (n/m)xp mm.

(b) Microscope with mechanical stage To attain the utmost exactitude with greater

facility, a microscope with mechanical stage should be used. The stage micrometer should be lifted by a metal wedge 1 or 2 cm in height. The fixed (lower) end of the stage micrometer rests on an S-shaped piece of tin which prevents it from moving. The metal wedge, being fixed by the spring of the mechanical stage, alters the angle of inclination of the micrometer with every movement of the mechanical stage. I t is thus very simple to find the relation of the figure on the mechanical stage to the values of the stage micrometer. If this relation is represented by a diagram, it will be available for all future references.