In studying about microscopes, it is imperative for the student to have a thorough knowledge about the terms used, like compound light microscope. How these words came about? The term compound refers to the microscope having more than one lens, whereas simple microscope like the one used by Leeuwenhoek had only one lens. The term light refers to the method by which light transmit the image to the eyes. Micro as we all know refers to small and scope means view.
Early microscopes, like Leeuwenhoek’s, were called simple because they only had one lens. Simple scopes work like magnifying glasses that you have seen and/or used. These early microscopes had limitations to the amount of magnification no matter how they were constructed.
The creation of the compound microscope by the Janssens helped to advance the field of microbiology light years ahead of where it had been only just a few years earlier. The Janssens added a second lens to magnify the image of the primary (or first) lens.
Simple light microscopes of the past could magnify an object to 266X as in the case of Leeuwenhoek’s microscope. Modern compound light microscopes, under optimal conditions, can magnify an object from 1000X to 2000X (times) the specimens original diameter.
In its simplest form - as used by Robert Hooke, for example - the compound microscope would have a single glass lens of short focal length for the objective, and another single glass lens for the eyepiece or ocular lens. Modern microscopes of this kind are usually more complex, with multiple lens components in both objective and eyepiece assemblies. These multi-component lenses are designed to reduce aberrations, particularly chromatic aberration and spherical aberration. In modern microscopes the mirror is replaced by a lamp unit providing stable, controllable illumination.
The optical components of a modern microscope are very complex and for a microscope to work well, the whole optical path has to be very accurately set up and controlled. Despite this, the basic optical principles of a microscope are quite simple.
The objective lens is, at its simplest, a very high powered magnifying glass i.e. a lens with a very short focal length. This is brought very close to the specimen being examined so that the light from the specimen comes to a focus about 160 mm inside the microscope tube. This creates an enlarged image of the subject. This image is inverted and can be seen by removing the eyepiece and placing a piece of tracing paper over the end of the tube. By careful focusing a rather dim image of the specimen, much enlarged can be seen. It is this real image that is viewed by the eyepiece lens that provides further enlargement.
In most microscopes, the eyepiece is a compound lens, which is made of two lenses one near the front and one near the back of the eyepiece tube forming an air separated couplet. In many designs, the virtual image comes to a focus between the two lenses of the eyepiece, the first lens bringing the real image to a focus and the second lens enabling the eye to focus on the now virtual image.
In all microscopes the image is viewed with the eyes focused at infinity (mind that the position of the eye in the above figure is determined by the eye’s focus). Headaches and tired eyes after using a microscope are usually signs that the eye is being forced to focus at a close distance rather than at infinity.
Connecting a digital camera to a light optical microscope

Microscope with LM digital adapter and Canon EOS 350D mounted to a phototube (C-mount thread), and Olympus E330 attached to an ocular tube.
To capture digital microscope images with a digital SLR camera, the digital camera must be optically and mechanically adapted to the microscope. An adapter connects the camera with the microscope. A firm mechanical connection is particularly important, because even the smallest movements (vibrations) of the camera strongly reduce the image quality. Furthermore, the light path must be optically adapted so that a fully lit, focused image is projected to the camera sensor (CCD/CMOS). There are several methods for attaching a digital camera to a microscope. One solution is to use the phototube. Using the adapter, the digital camera is screwed firmly onto the tube. The two oculars continue to be used for the visual observation of the specimen. Unfortunately, almost all microscopes that are equipped with a phototube are very costly. For simple purposes, another option is to directly place a digital camera, without any adaptations, directly to the ocular, and to capture an image with a steady hand. Due to the lack of optical adaptation, however, this method produces a smaller, vignetted image in most cases. Vignettation means that the edges of an image are darker than the centre. This effect causes only a small part of the sensor to be optimally used; the rest remains black. A more professional, but also more costly solution is to use a tube adapter. With this method, the ocular is removed and an adapter is fitted into the phototube with the digital camera. The adapter acts as a mechanical and optical interface between microscope and digital camera. This makes it possible to avoid motion blurs due to camera shake and vignettation effects, leading to a much higher quality of the image. Original Article