It may seem odd and an apparent contradiction since viruses cannot be seen by the naked eye and can only be examined under a high power microscope. Most of these viruses are beyond the range of even the best microscope. The limit of size of the particle which we can see with an ordinary microscope is governed by the wave-length of visible fight. This is a fixed law which cannot be altered. Therefore since the limit of resolution of the microscope is about 200 mµ (one may equal to one millionth of a millimetre), an object must be at least 250 mµ in diameter to be properly visible. We have learned that the measurements of viruses differ. Most of the viruses are below 100 mµ and some are less than 20 mµ in diameter.

For these very minute, seeing viruses using an ordinary microscope is not going to help much. However, at present this is a wide range of high power microscope models to choose from that would meet the different demands of laboratory experiments. Viruses can also be studied by using invisible light, or what we call as the ultra­violet light. Ultra violet light has a very short wave-length and thus allows much smaller particles to be resolved. However, if the ultra-violet rays are invisible, how does that help in obtaining an image of a virus? The answer to this question is by means of the camera. What the eye cannot see, the photographic plate can.

It is therefore possible to obtain a photograph of a virus using this method. It has to be done with a very special high power microscope fitted with quartz lenses and with a camera attachment. Using this method Barnard has succeeded in photographing several animal viruses, including vaccinia virus which measures about 200 mµ in diameter as well as the virus of fowl plague which measures about 100 mµ in diameter. Photographs of these viruses show numerous round bodies very similar in appearance to minute round bacteria, or what is commonly called as cocci. However, in spite of the very short wave-length of ultra­violet light, the limit of resolution obtainable by this means is only about 100 mµ. With this we are still unable to get a photograph of the smaller viruses just like that of the foot­ and-mouth disease and those viruses affecting plants. But that is not the way of scientific research. If there is an insurmountable object in the way, the scientist or investigator must find ways and means to go around it. That is what has been done in this case. If the wave-length of light, visible or invisible, does not allow us to resolve the smaller viruses so that they can be photographed, then something else must be done and that is the substitution for the light. After several trails and errors, the physicists have produced a high power microscope known as the Electron Microscope which does not use light rays at all but operates with cathode rays. Instead of a beam of light it employs a stream of electrons as well as electromagnets instead of lenses. Now the wave-length of visible light is of the order of a tenth of a micron (one ten-thousandth of a millimetre), but the wavelength of the electron beam goes down to a thousandth of a millimicron (one thousand millionth of a millimetre) according to the high tension used, i.e. voltages of between 60,000 and 80,000. The electron microscope has the capacity to obtain resolving power of about 5 mµ which will cover all the smallest viruses. This is how important this electron microscope is in the study of the viruses.

A number of viruses have already been photographed by means of this new high power microscope and interesting results have been obtained. One of the first viruses studied was that of tobacco mosaic. The photographs at once confirmed the suggestions, based on other methods, about the size and shape of this virus. They showed that it was a long rod about 300 mµ in length and about 15 mµ broad. The photographs likewise confirmed a previous belief that these rods were inclined to stick together end-to-end to form long chains. Another virus studied was the tomato bushy stunt. Again, further evidence was obtained that the virus of tomato bushy stunt was a very small sphere, a fact already suspected from the results of other methods of study.

With the use of high power microscopes such as the electron microscope studies about the viruses using different methods further confirmed the evidences concerning the shapes and sizes of virus particles and many others. The electron microscope is a great help to scientific researchers.