Electron Microscope Comparison

Electron Microscope

The microscope with glass lenses or glass lenses combined with lenses of natural transparent minerals like fluorite or quartz may be so constructed as to give almost any degree of magnification and thus make minute objects visible to the eye. However, for the showing of details as separate things, that is, for resolution, such a microscope is limited by the length of the waves of light to a magnification of about 1500 diameters.

Hence investigators, knowing that details beyond the ability of the ordinary microscope to reveal are probably even more pervasive than those visible with the best microscopes, have sought radiations of shorter wave lengths than those of visible light, hoping thereby not only to add to the magnification but to produce the corresponding increase in resolution.

It is believed that in the newly-invented electron microscope using electronic waves 1/100,000th of the wave-length of visible light, 25,000 to 100,000 magnification with accompanying resolution can be attained. This, of course, would make possible an almost unbelievable exploration of minute objects with their details, and it may be added, corresponding difficulties in interpreting the appearances.

The possibility of making such a microscope depends upon the fact that the electronic waves are propagated in straight lines like light waves and that they may be concentrated and focused by electro magnets something as light is concentrated and focused by glass lenses. Also, depending on the strength of the current in the electro magnet, the variable concentration and focusing are comparable to the concentration and focusing in the light microscope by means of different curvatures of the glass lenses.

As the electronic waves are wholly invisible, their images must be made visible either by using a fluorescent screen or by means of a photographic plate. Therefore the electron microscope is to be com pared with a projection microscope in which the image formed is entirely independent of the eye. It is not like the microscope into which one looks, for with this the eye of the observer forms a part of the optical train, and the final image is formed upon the retina of the eye.

In the accompanying diagrams the constituents of an electron and of a projection microscope are shown for comparison; an electron source in one and a light source in the other; for illuminating the object a magnetic condenser in one and a glass condenser in the other; for producing a real image of the object, a magnetic objective and a glass objective respectively; and finally a magnetic projector and a glass projection ocular to form the screen image in each case.

The size of the final screen image may differ greatly in the two instruments, but varies most with the electron microscope, in which it may be as great as X25 while with the light microscope it rarely exceeds X10 or X15.

In the diagram the projection microscope is represented as producing a screen image of X100X10 = X1000 and the electron micro scope X100X60 = X6000; but, as has been said, their respective possibilities are as great as about X1500 for the light microscope and X25,000 for the electron microscope; and this magnification, in some cases at least, might be increased to X100,000 in printing the negative.

There are three other differences that should be mentioned:
(1) The initial cost of the best ordinary microscope is in the hundreds while that of the electron microscope is in the thousands of dollars.
(2) All parts of the ordinary microscope can be used in the air of any room while all the effective parts of an electron microscope must be in as complete a vacuum as possible.

(3) With the ordinary microscope the object can be of considerable size and thickness, and mounted upon a glass slip in air or some transparent medium like glycerol or Canada balsam. With the electron microscope the object must be small, very thin and dry and mounted on a film of collodion, which in turn is supported by a perforated disc of platinum or a fine wire screen.

It will be seen from the above that at present the electron microscope is not available for ordinary biological study, but is to be welcomed for the uses to which it is adapted now and for the possibilities in its future development.

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