Microscopy

The ultra-microscope has been used for many purposes, such as obtaining estimates of the number of smoke particles in the air of different localities, for estimating the amounts of foreign matter in water or in glues, gelatines, etc., as well as for many purposes in connection with the study of colloids. The instru ment is also used in investigating the structure of natural and artificial fibres, for the examination of artificial fibres for inclu sions, etc. Although the ultra-microscope fails to reveal particles which are separated by distances smaller than the limit of reso lution of the object-glass used, the brightness of the haze of light obtained affords a measure of the size and concentration of the particles. Increase of brightness indicates either increase in the number of particles per unit volume or increase in the size of the individual particles. This effect can be made use of in certain work on very minute living organisms and in the study of the growth of minute crystals separating out from solutions. The microscope when used for this kind of work is being employed as a Nephelometer.

Ultra-violet Microscopy.

For the microscopic examination of objects having structures too fine to be resolved by any object glass when ordinary visible light is used, recourse must be had to methods involving the use of ultra-violet radiations and of lenses made of fused quartz computed for use with ultra-violet light of appropriate wave-length. Glass lenses are opaque to these radiations. In 1904, Dr. Kohler of Jena experimented with ultra violet light with a view to obtaining resolution of structures previ ously irresolvable. He used monochromatic radiation of wave length 275 p.p.given by an electric arc between cadmium poles, and projected the image on to a fluorescent screen for visual examina tion or for focussing in readiness for photographic recording.

In England J. E. Barnard has developed a technique for ultra violet work which is much simpler than that used by Kohler. In his more recent work described in 1925, he used a combined illumina tor made by the firm of R. & J. Beck consisting of an outer sys tem of glass, which acts as an immersion dark-ground illuminator, and an inner immersion system of quartz which enables a transmitted beam to be passed through the object. Both con densers have the same focus, the one for visible light and the other for the ultra-violet radiation used. Either condenser can be stopped out at will, the quartz one by means of a central stop and the glass one by means of an annular diaphragm. The con denser is put in immersion contact with the quartz slide on which the object is mounted, and the object is illuminated with visible light by means of the dark-ground illuminator. The microscope

is then focussed so that the object is clearly seen by visible light. The dark-ground illuminator is then closed, the central stop is removed so as to uncover the quartz condenser, and a beam of ultra-violet light is passed into this condenser and focussed by it, without further adjustment, on to the object. The object-glass has not the same focus for ultra-violet light as for visible light, however, so it is necessary to readjust the focus before any photo graph can be taken with the ultra-violet light. Arrangements are provided so that by moving the fine adjustment through a known small distance, the requisite readjustment of focus can be made with certainty. If desired, a number of photographs of one object can be taken in succession, so as to show the structure of the object in successive parallel planes separated by distances of the order of ,000 inch.

The technique developed by Barnard has brought ultra-violet microscopy into an important position in bacteriological research. Bacteria are nearly uniformly transparent to visible light but are not so to certain regions of the ultra-violet spectrum. To photo graph bacteria it has hitherto been necessary to stain them with dyes in order to make their outlines or structure visible with transmitted light. In the process of staining, the bacteria are killed, consequently any deductions drawn as to the form and structure of living bacteria are liable to be fallacious if based solely on the microscopic examination of stained specimens. With ultra violet illumination staining is no longer necessary. Thus, not only is it possible with this new technique to examine the structure of bacteria which have never previously been resolved, but also it is possible to avoid staining the bacteria, so that the structure of living bacteria, whether large or small, can be photographed.

The technique worked out by Barnard offers great promise in other directions, notably in metallurgical research, for determining the detail-structure of metals and alloys. Experimental work to explore the further possibilities of ultra-violet microscopy is in progress, investigations have also been suggested with a view to discovering whether the very short ultra-violet radiations, of wave lengths about 4o to 6o AA can be used in microscopy. These Schumann rays are absorbed to a considerable extent by all forms of matter, a layer of air one or two centimetres thick being suffi cient to absorb them almost completely, consequently any appara tus in which such rays are employed must be used in a vacuum and must be of the reflecting type.