The Electron Microscope as Ultramicroscope

 

Ernst Ruska, Dr. Of Engineering

Technical High School, Berlin

Scientific Research Papers Published In The Bimonthly Journal Forschungen und Fortschritte - Research And Progress, Edited By Dr. Karl Kerkhof

Volume I - January 1935 - Number 1

Theoretical and experimental research hitherto undertaken in the optics and microscopy of electrons has shown that a beam of cathode rays obeys geometric optic laws in just as exact a manner as rays of light. The optical elements, such as mirrors, lenses, and prisms, can be replaced by suitable electric or magnetic fields, through which the cathode ray passes. The value of the electron microscope for scientific and technical research lies in two directions. In both ways the electron microscope can open up fields of knowledge hitherto closed to us.

In the first stages, as long as the electron lenses had not been much investigated, the possibilities of application lay in revealing the distribution of cathode rays in space and time. For this purpose moderate degrees of magnification are sufficient (magnification 100 fold, linear), since the smallest units of radiation distribution to be observed are in the region of 1 to 10 m. The second and, from the point of view of scientific knowledge, more important branch of research, which until recently had not been undertaken, is characterised by the fact that the wavelengths attributed by de Broglie to the electrons are several orders of magnitude less than those of visible light. Herewith the limits of geometrical optics are correspondingly extended by means of diffraction phenomena, that is, a microscope working with this ultrashortwave light can examine particles which are several orders of magnitude smaller than the smallest that are visible through an ordinary microscope (about 0.3 m). An electron ray microscope, which is to detect minute particles of 10^-4 to 10^-5 mm, must have a linear magnification of about 10,000 when the final real image clearly visible on the screen is to be l/10 mm. In contrast to the previously mentioned cathode experiments, these latter investigations are of interest for the examination of all sorts of objects, such as metal foils, very fine fibres, and organic objects from the spheres of medicine and biology.

With the support of the Notgemeinschaft der Deutschen Wissenschaft - German Scientific Union and the Gesellschaft der Freunde der Technischen Hochschule, Berlin - Society Of Friends Of The Technical High School, Berlin, I have therefore carried out experiments in such a way that the test objects (1 m thick aluminium foil as well as fibres of cotton and artificial silk) were penetrated by strong rays of electrons moving at 60 to 80 thousand electron volts. The images of the foils, after two stages of magnification by means of magnetic coils of short focal distance, were made visible on the screen and photographed. The radiation was intensified by means of a condenser coil which collected on the object the electron rays emitted from the cold cathode of an ion tube. The objects were fixed between two copper diaphragms, which were finely drilled through (0.3 mm) in the centre. Eight such object carriers were fixed so that they could move around in a circle, and could be brought one after the other into the path of the rays from the vacuum tube. Magnifications of over 12,000 fold linear were obtained. For 8,000 fold magnifications, changes from light to dark in the final image within a range of 0.2 to 0.3 mm. The experiments made up to the present have shown clearly that an increase of magnifying power up to 20,000 and 30,000 is only possible with the means and methods known today. The magnetic coil seems to be a very suitable lens (because sufficiently free from defects) for such extreme magnifications. The difficulties in the experiment lie in the loss of heat by the object, in the not absolutely constant radiation of the electron source, and in small mechanical vibrations of the apparatus.

The last two circumstances make the taking of the photograph especially difficult, while the visual observation of the images remains unaffected. What is extremely convenient is the ease with which the intermediate image can be observed, whereby one can at any time get a view of a considerable field.

  

Since the electron microscope is of such fundamental importance for ultramicroscopy, it is desirable that medical men and biologists as well as physicists should take part in the research work which still remains to be done.