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The Lateral Chromatic Aberration of Apochromatic Microscope Systems

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The Lateral Chromatic Aberration of Apochromatic Microscope Systems RP316 THE LATERAL CHROMATIC ABERRATION OF APOCHRO- MATIC MICROSCOPE SYSTEMS By I. C. Gardiner and F. A. Case ABSTRACT Methods are described for the measurement of the lateral chromatic aberration and distortion of microscope objectives and eyepieces. Measurements have been made on 20 apochromatic microscope objectives of four different makes, and on 10 compensating eyepieces of two makes. The results show that the compensating eyepieces, in general, do not entirely compensate the chromatic aberration of the objectives, and that the complete microscope system is usually undercorrected. The bearing of this on the performance of the system in photo- micrographic work by a 3-color process and in the direct visual observation of objects is considered in detail, CONTENTS Page I. Introduction 937 II . Chromatic aberration of the objectives 938 III. Chromatic aberration of the eyepieces 940 IV. Chromatic aberration of the complete optical systems 944 I. INTRODUCTION For the apochromatic microscope objective the images produced by light from different parts of the spectrum differ in size. As a consequence, at all points other than the center of the field, the images do not register, the lack of registry increasing with increase of distance from the center of the field. Such a defect is termed lateral chromatic aberration. This aberration is designedly left in the objective because, by this means, a more perfect correction of spherical aberration for the entire range of the visible spectrum may be secured. The lateral chromatic aberration in the image, produced by the objective, is then compensated by the eyepiece which introduces a lateral chromatic aberration of the opposite sense, thus giving origin to the designation " compensating eyepiece." For the objectives of shorter focal length and greater numerical aperture the limitations of the optical materials available prescribe the amount of lateral chromatic aberration present. For the objectives of longer focal length and smaller aperture ratio, the lateral chromatic aberration, although it could be more perfectly corrected without detriment to the correction for spherical aberration, is made the same in amount as that for the objectives of greater numerical aperture in order that all of a series majr be used inter- changeably with a single series of compensating eyepieces. For very precise work it may then become a question whether or not the lateral chromatic aberration is held sufficiently constant for the different focal lengths to give the desired freedom from color and also whether or not the lateral chromatic aberrations of objective 937 Bureau of Standards Journal of Research [Vol. 6 permit the interchange- and eyepiece are sufficiently standardized to one manufacturer with an eyepiece manu- I an objective of v another. The present investigation is the result of a A this character which arose in connection with the making reproduction at the U. S. Army , 3-color i ohs for urn. In such work three photographs are taken suc- ely through Eastman photographic filters, A, B, and C, which ! approximately to wave lengths 620, 550, and 450 rn.fi, respectively, and from these three photographs the three plates are made for produc- ing the impressions in three different colors which form the final color print. Such work constitutes a more se- vere test of the color cor- rection of a system than does the more conventional use of the microscope, since a very slight lack of regis- ter in the color print may be detected and is consid- ered objectionable. It should be mentioned that this use does not strictly accord with the conditions for which the microscope system is designed. When the microscope is used for the direct visual examina- tion of an object, the image formed by the eyepiece is virtual, whereas when used for photographic purposes the image is real. The dif- - ngi ,„cn i for testing the objectives ference arising from this va . riation in use is probably not ; jause in both cases the image distance is large in com- tfith the focal length of the eyepiece. II. CHROMATIC ABERRATION OF THE OBJECTIVES ,l ' ";' ". applied to the objective is illustrated in Fig- !' \ ><1 ("ot indicated in the figure) was adjusted ^e tube r in a vertical position and the objective to be *ted in the usual position at N. On the stage& of the "as aced the P object S, which was a glass slide carrying ' l':";'U< l""'s, ; spaced 0.1 mm ruled in silver by means of ! """ arc at J, the ; ^ condenser lens L, and the mirror '"•"•V B, and C filter, which were mounted in illuminating system. The photographic plate rested on the upper end of the draw d to bring (he plate in the image plane cor- — Gardner] Case J Chromatic Aberratio7i of Microscopes 939 responding to a tube length of 160 mm. Stray light from the r< was excluded by the brass cup K, mounted on the upper end of the microscope tube, and by the wooden plug A, to which the plate loosely attached in order to facilitate its removal from the brass cup after the exposure. For each objective three negatives were obtained, one corres- ponding to each of the three filters. The spacing of the lines in the image, as recorded on the negative, was measured on a comparator and the magnification obtained by dividing the distance between two lines on the negative by the distance between the two homologous lines on the object slide. Tins comparison of distances was made each pah- of adjacent lines recorded on the plate. If there were an appreciable amount of distortion the values of the magnification thus obtained for any one lens would have changed systematically, either increasing or decreasing, as pairs of lines were selected farther from the center of the field. In none of the objectives tested, was such a progressive change found, and it is concluded that the distortion present is substantially less than that corresponding to a variation in magnification of 0.1 per cent for the different parts of a field 18 mm in diameter. Table 1 gives the results of measurements which have been made on 20 objectives of four different makes. Table 1. Magnification ratios of objectives tested Magnifica- Magnifica- Refer- tion, B filter tion, ( Fecal length Magnifica- ence Make divided by divided by (mm) tion, B filter No. magnifica- magnifica- tion, A filter tion, A filter 1 I 4 - 33.9 1.005 1. 014 2 I 8 16.4 1.005 1. 014 3 I 8. 16.6 1.005 1.012 4 I 16- 8.2 1.005 1.014 5 I 16- 7.8 1.006 1.015 6 I 1.005 1.014 7 II 4 39.2 1.006 1.015 8 II 4 37.0 1.005 1.016 9 II 8 . 16.5 1.006 1.017 10 II 8. 16.8 1.005 1.016 11 II 16. 8.3 1.004 1.013 12 II 16. 8.2 1.006 1.014 13 II 16. 8.1 1.005 1. 013 14 II 1.006 1.015 15 III 3.. 45.5 1.003 1.017 16 III 4 35.9 1.007 1.019 17 III 8 . 17.4 1.004 1. 012 18 III 16. 8.1 1.006 1. 020 19 III 16. 9.0 1.006 1.018 20 III 1.005 1.017 21 IV 2 57.7 1.000 1.007 22 IV 4 36.0 1.005 1.013 23 IV 16- 8.7 1. 005 1.013 24 IV 1.003 1.011 The fourth column gives the magnification for the B filter, the fifth the ratio of magnification for B filter to the magnification for A fi] and the sixth column the ratio of magnification for C filter to magni- fication for A filter. If there were no primary lateral chromatic Journal Research [vol. e 940 Bureau of Standards of of the size aberration the A and C images would be same and the Reference to these ratios in the sixth column would be 1. smaller than the B image which, in turn, is - that the A image is of a white point source not smaller than the C image. The image therefore, be a short spectrum, on the axis of the objective will, turned toward the center. placed radially in the field with the red end of these apochromatic objectives were used to produce an 250 mm in diameter with no compensation introduced by the eyepiece, then for the A and C P images of a point at the edge of the picture the failure to register would be of the order of 2 mm. This E illustrates the importance of compen- sation and indicates why the prom- I inent colored fringes are observed when the apochromat is used with an eyepiece which is not compensating. Reference to the average values for the different makes shows that III and IV differ most in the character of compensation which their objectives offer. Again, using a projected image 250 mm in diameter as an illustra- a tion, an eyepiece which compensated an objective of Group III perfectly, might, if used with an objective of Group IV, give A and C images which fail to register at the edge of the picture by 0.8 mm. d III. CHROMATIC ABERRATION OF THE EYEPIECES The arrangement for testing an eyepiece is shown in Figure 2. The source of light was a diffusing screen of opal glass at B, illuminated by a cluster of incandescent lamps near the focus of a concave reflector, R, built up from small pieces of plane mirrors mounted in a frame.
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