The remaining color seen in an image formed by such a lens is said to be secondary chromatic aberration. A truly achromatic lens would transmit light without decomposing it spectrally and there would be no color fringing in the image. A doublet, composed of a positive and negative lens element, can be made achromatic for two colors which much improves the performance for most of the others. The simplest achromatic is a doublet that combines two single lenses with different dispersions and curvatures to achromatize the combination. An intermediate, enclosed chamber of a vacuum or pressure system through which an object may be passed without effectively changing the vacuum or pressure of the main body of the system.
Reference here is to the vacuum of all electron microscopes.
Airy disk the image of a bright point object as focused by a lens system. With monochromatic light, it consists of a central point of maximum intensity surrounded by alternate circles of light and darkness caused by the reinforcement and interference of diffracted rays. The light areas are called maxima and the dark areas minima. The distribution of light from the center to the outer areas of the figure was investigated mathematically by Sir George Airy. The diffraction disk forms a basis for determining the resolving power of an ideal lens system.