The Compound Microscope
A compound microscope is one in which magnification is achieved in two stages. It utilises an objective lens -- usually several of different power on a turret -- and an eyepiece that is used to further magnify the image formed by the objective lens. Most modern compound microscopes have binocular heads so that both eyes can be used to view the specimen. And many of them have an extra tube to which a camera (film, digital or video) can be attached. Modern objectives are not single lenses. They consist of a number of optical elements. Some might have eight separate lenses some of which are made of fluorite instead of glass. These serve to improve the image by correcting aberrations and reducing the curvature of the field (flatten the image) which is desirable for photography. The eyepieces are also multi-element lenses, which, in combination with their matched objectives, further improve the image. The image obtained is reversed left and right and upside down. A condenser is a very important part of the transmission light microscope. This device focuses the light on the specimen from below and also needs to be highly corrected to prevent coloured fringes and to spread the light evenly across the field.
A compound microscope fitted with a Wild automatic 35mm camera.
There are four Leitz Fluotar objectives on the turret with magnifications from 10X to 100X.
Illumination of the specimen can be done in two ways. From below, in which case the light passes through the specimen (transmission) or from above (incident). Biological specimens are usually viewed by transmitted light whilst Materials Scientists and Geologists most frequently use incident light.
The UV Microscope
This is a compound microscope in which all the optical elements are made of quartz. A Mercury arc or Xenon burner is used as the light source and the short wave ultra-violet light allows resolution of about twice that possible with visible light. All the optical elements, including the specimen slide and cover glass, are made of quartz which allows UV light to pass. Special interference filters remove all the visible light. The image is formed on photographic film or a digital sensor array. UV is extremely harmful to the human eye and the image would be invisible in any case.
The Fluorescence Microscope
This is another modification of the compound microscope that uses UV light to excite fluorescence of the specimen. Many natural substances will fluoresce under UV light. In addition special dyes which have an affinity for certain structures in biological specimen can be used to stain the specimen. These dyes gives off characteristic fluorescence when irradiated with UV light. The dyes often fade rapidly and this complicates the method. The specimen may be illuminated from above in which case it is described as epi-fluorescence or below as trans-fluorescence. Special filters keep the harmful UV light from reaching the eyes of the observer.
The Confocal Microscope
The Confocal microscope employs a fine laser beam which is scanned over the specimen and the image is built up point by point rather like that in a TV monitor. The instrument is similar in many respects to an upside down compound microscope, but uses a very narrow scanned laser beam to image the specimen. The beam is scanned by means of vibrating mirrors and the transmitted dot of light is measured by a detector behind a small aperture. The modulated beam serves to build an optical section of the specimen and living, as well as fixed and stained, biological material can be viewed. The Confocal instruments are invariably used combined with fluorescence and several lasers emitting different wavelengths are built into the latest designs.
An Olympus Confocal microscope being used to view a living cancer cell culture.