Modern compound microscopes are designed to provide a magnified two-dimensional image that can be focused axially in successive focal planes, thus enabling a thorough examination of specimen fine structural detail in both two and three dimensions.

Introduction - The optical components contained within modern microscopes are mounted on a stable, ergonomically designed base that allows rapid exchange, precision centering, and careful alignment between those assemblies that are optically interdependent. Together, the optical and mechanical components of the microscope, including the mounted specimen on a glass micro slide and coverslip, form an optical train with a central axis that traverses the microscope stand and body.

The microscope optical train typically consists of an illuminator (including the light source and collector lens), a substage condenser, specimen, objective, eyepiece, and detector, which is either some form of camera or the observer's eye. Research-level microscopes also contain one of several light-conditioning devices that are often positioned between the illuminator and condenser, and a complementary detector or filtering device that is inserted between the objective and the eyepiece or camera. The conditioning device(s) and detector work together to modify image contrast as a function of spatial frequency, phase, polarization, absorption, fluorescence, off-axis illumination, and/or other properties of the specimen and illumination technique. Even without the addition of specific devices to condition illumination and filter image-forming waves, some degree of natural filtering occurs with even the most basic microscope configuration.

Examine the internal components of the optical microscope with these cutaway diagrams of popular teaching and research microscopes. Each illustration is extensively labeled to aid understanding of the placement and proximity of lenses, diaphragms, mirrors, filters, shutters, lamps and other components contained within a modern microscope.

Olympus BX51 Research Microscope - Featuring advanced Olympus UIS infinity-corrected optics, the BX51 is designed for both transmitted and reflected light utilizing a variety of contrast enhancement techniques including fluorescence, differential interference contrast, phase contrast, and darkfield illumination.

Nikon Eclipse E200 Student Microscope - The Eclipse E200 student microscope is equipped with the CFI60 optical system, which is a combination of Nikon's CF specialty optical glass alloys utilized in objective construction coupled to internal microscope lenses to produce a unique infinity-corrected system. This advanced optical system provides long working distances, high numerical apertures and flat images over the entire field of view with virtually no curvature of field when the field number is 20 millimeters or lower.

Olympus BH2 Research Microscope - Standard equipment on this research-level microscope included 10x widefield high eye point eyepieces having a field number of 20, precentered halogen lamps, aspherical collector lenses, and fully enclosed light paths designed to exclude dirt and dust. Auxiliary components for the microscope included photoeyepieces, an automatic exposure photomicrography system, and a wide range of objectives covering the entire range of correction and magnification.

Geometrical Construction of Ray Diagrams - A popular method of representing a train of propagating light waves involves the application of geometrical optics to determine the size and location of images formed by a lens or multi-lens system. This tutorial explores how two representative light rays can establish the parameters of an imaging scenario.

Perfect Lens Characteristics - The simplest imaging element in an optical microscope is a perfect lens, which is an ideally corrected glass element that is free of aberration and focuses light onto a single point. This tutorial explores how light waves propagate through and are focused by a perfect lens.

Perfect Two-Lens System Characteristics - During investigations of a point source of light that does not lie in the focal plane of a lens, it is often convenient to represent a perfect lens as a system composed of two individual lens elements. This tutorial explores off-axis oblique light rays passing through such a system.

Projection and Viewing Eyepieces - The eyepiece (or ocular) is designed to project either a real or virtual image, depending upon the relationship between the intermediate image plane and the internal eyepiece field diaphragm. Explore how eyepieces can be coupled to the human eye or a camera system to produce images generated by the microscope objective.

Condenser Image Planes - In a microscope optical system, the lamp filament is imaged in the focal plane of the condenser aperture diaphragm when the microscope is configured to operate under conditions of Köhler illumination. This tutorial explores the relationship between image planes relevant to the field and condenser diaphragms and how aperture size affects ray trace pathways.

Microscope Conjugate Field Planes - In a microscope optical system, the lamp filament is imaged in the focal plane of the condenser aperture diaphragm when the microscope is configured to operate under conditions of Köhler illumination. This tutorial explores the relationship between image planes relevant to the field and condenser diaphragms and how aperture size affects ray trace pathways.

Infinity Microscope Conjugate Field Planes - The geometrical relationship between image planes in the optical microscope configured for infinity correction with a tube lens is explored in this tutorial. In such a microscope, magnification of the intermediate image is determined by the ratio of the focal lengths of the tube lens and objective lens.

Olympus BX51 Microscope Light Pathways - Explore the optical pathways for illumination in the Olympus BX51 research-level microscope. This interactive tutorial contains sliders and radio buttons that allow the user to adjust the intensity of illumination and to adjust a virtual beamsplitter that directs light to the eyepieces or a camera system.

Selected Literature References - The list of reference materials provided in this section attempts to cover both early reviews and the latest information available on microscope components and the microscope optical train. Many references are comprehensive and cover a majority of topics concerning lenses, principal planes, off-axis light rays, virtual images, and image planes, while other references explore these concepts in conjunction with topics such as image formation, optical aberrations, and illumination.

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