Page:Principles of Microscope.djvu/139

Rh

Set up, as in Experiment 4, subsect. 2 supra, in front of the window a microscope provided with an Abbe condenser, and bring into view under a 1-inch or higher objective the image of the window bar which is furnished by the condenser. This will be seen fringed (as shown in Plate II a and Plate III d) with coloured bands.

Place upon the stage of the microscope a film preparation of tubercle bacilli mixed with other bacteria and stained differentially as shown in Plate II d.

If the objective has not been properly corrected for chromatic aberration it will be found that the bacteria which are coloured in red and blue respectively are imaged, not as in the figure, on one and the same focal plane, but the bacteria which are stained red on a sensibly higher plane than the bacteria which are coloured blue.

Hold up to the eye an ordinary high-power ocular. Note that the aperture of the diaphragm which represents the aperture of the combination is margined by a faint blue band.

5. Method of correcting for chromatic aberration.

While chromatic aberration can be abolished only by the employment of monochromatic light, more or less complete corrections can be effected by combining convex and concave lenses of different refractive indices.

What is aimed at in the case of the ordinary—so-called achromatic objective—is a bringing together of the foci of the red and blue rays, in particular, a bringing together of these foci in the case of the beams which occupy the centre of the field.

In the case of apochromatic objectives of Zeiss, a higher degree of chromatic correction is attained. Here the maker aims not only at the complete synthesis of the foci of the blue and red rays which lie at opposite ends of the spectrum, but also complete synthesis of the foci of the rays which occupy an intermediate position in the spectrum, and further he aims at the achievement of this correction over the whole area of the field.