SOME REMARKS ON THE IMAGE CONTRAST IN ELECTRON MICROSCOPY AND THE TWO-COMPONENT OBJECTIVE

Abstract

An image produced by a microscope must produce a sharp geometrical image with adequate contrast. With the electronic mi- croscope, a monochromatic instrument, contrast can produced only through variations in intensity, by variation in the electron-scattering power of different objects. An electron microscope objective is operated in one of 2 ways, with a large or a small physical aperture, using either spherical aberration or "diffraction defect" to focus. To overcome practical difficulties a new lens was designed consisting of 2 components. The first is a weak component that has a focal length of about 11 mm. and is situated 22 mm. from the specimen. Thus this lens produces a real image at a magnification of unity. The 2d component is a short focal length lens of the same design as a convention electron microscope objective. The real image produced by the 1st component is situated very nearly at the focal point of this lens, and hence the lens produces a 2d real image at a relatively high magnification (150 X) that corresponds closely to the image produced by a standard objective. Thus this new objective has essentially the same focal length as a standard objective but has a working distance ten times as long. Since the numerical aperture is the ratio of the radius of the physical aperture to the working distance, we are able to use apertures that are still 50 [mu] in diam. but that result in numerical apertures 10 times smaller than would be obtained with the same aperture in a conventional lens. This very small numerical aperture provides a greatly increased contrast but also results in an appreciable reduction in the resolving power. Fortunately this is still adequate for many biological problems. It should be pointed out that this lens has been used strictly for exptl. purposes and is not recommended for general use in the electron microscope. Its spherical aberration is as much as 50 times greater than a conventional objective. Its chromatic aberration is as much as 10 times greater; similarly increased is its sensitivity to changes in the exciting current of the objective.