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ULTRAMICROSCOPE. An optical ap paratus designed for the collective study of particles that are too small to be individually seen by methods formerly in use. It is not, properly speaking, a new instrument, but is merely an ordinary microscope provided with certain accessory devices for illuminating the field in a special way. It is well known that the possibility of seeing small objects by means of optical magnification is limited by the fact that light has a finite (though short) wave length. The theory of the telescope indicates that the resolving power of the instrument that is, its ability to reveal small details — in creases in direct proportion to the diameter of the object glass. Similarly in the case of the microscope, theory shows that there is a limit to the fineness of the detail that can be seen, and it is known that this limit is determined by the numerical aperture of the objective and the wave-length of the light that is used—the re solving power being greater for high apertures and for short wave-lengths. In consequence of this theoretical limitation, it follows that no microscope, however perfect, can show an ob ject in its true form and size, unless the di ameter of the object is greater than about half the wave-length of the light by which it is viewed. In the case of an exceedingly small object diffraction phenomena become very marked; and when the diameter of the object is less than the limit indicated, these phenom ena become so pronounced that the objective can no longer form a definite magnified image, corresponding in shape, size or details with such configurations as the real object may pos sess. If the particle to be observed is station ary, it may be photographed by means of ultra violet light; and it is possible, by this means, to obtain somewhat more detail than can be per ceived by the eye, which is sensitive only to light of a longer wave-length. No great gain can be made in this way, however, because there is a limit to the shortness of the of the ultraviolet light that can be used, and the most that we can reasonably hope for, in this direction, is to increase the visibility of de tail sufficiently to enable us to learn something of structures having one-half or the linear dimensions that can be directly perceived by the eye while using ordinary light. By means of the ultramicroscope, however, it is possible, under favorable conditions, to per ceive, by the eye, particles having linear dimen sions as small, for example, as 1 per cent of the wave-length of the light that is used. The form of these small particles i cannot be cerned, on account of the limitation by diffraction and noted above. They appear like circular discs, having an apparent diameter much greater than their real diameter. Yet notwithstanding the impossibility of ascertain ing the real form of these particles, i t is often useful to be able to demonstrate their existence and to count their number, ascertain their aver age mass and study their translatory move ments; and these things can be done by the aid of the ultramicroscope. The principle upon i which the ultramicroscope is based is exceed ingly simple. We see it exemplified in every day life when a strong beam of sunlight trav erses the air of a room that is otherwise dark or dim. As is well known, the beam, when ex amined from the side, reveals the existence of countless motes or particles floating in the air. Everyone of these brilliantly illuminated parti cles acts as a secondary source of radiation and hence can be perceived by the eye. Intent

examination of a beam of sunlight under these conditions will show that some of the par ticles present are large enough for their forms to be seen with a certain degree of defi niteness by the eye. There arc many others, however, that are altogether too small for this, and these smaller particles are perceived merely as luminous points, without visible form.

The principle of the sunbeam and the mote laden air is applied in the ultramicroscope with out essential modification. In the form in which the instrument is applied to the study of liquids and of such solid particles as they may hold in suspension, a narrow but powerful beam of light is sent through the liquid in a horizontal direction, while the liquid is examined from above by means of an ordinary microscope pro vided with a water-immersion objective — the optic axis of the microscope being at right angles to the direction of the beam of light. By the aid of an arrangement of this sort, par ticles that are altogether too small to be per ceived under ordinary conditions may be made to stand out like tiny luminous discs against a background that is either dark or at all events far less brilliant. In liquids, the particles that are thus seen exhibit active Brownian move ments (q.v.). The size of the particle that can be seen is determined by the brilliance of the il lumination. If the liquid under examination contains particles of every gradation of fine ness, then with any given intensity of illumina tion the particles that are too small to be seen in their true form, but which are nevertheless large enough to be separately revealed by means of this method of observation, appear as small luminous discs. Particles that are too small to be separately perceived in this way under the given conditions of illumination re veal themselves, collectively, by communicating a hazy appearance to the beam of light.

Lord Rayleigh had shown, in 1899, that a particle that is too' small to be separately seen by the microscope under ordinary conditions may, nevertheless, become visible if it is illu minated powerfully enough, even though it may be beyond the power of the microscope to re veal its true form. Dark-background illumina tion, in which the object is seen by reflected, re fracted, diffracted or polarized light, and not by ordinary light that is transmitted directly through it, has also been familiar to microscop ists for many years, and the Wenham para bolic substage condenser, in which the central rays are cut out by means of a central stop, while the oblique peripheral rays proceed up ward at such a degree of angularity that they cannot directly enter the objective, has been a familiar accessory since 1872. In ultramicro scopy the principles of darkground illumina tion are merely developed scientifically and pushed to their ultimate limit; and the results that are obtained by observing exceedingly small particles under such conditions are studied with corresponding care. The method was first applied to the study of colloidal solutions by Seidentopf and Zsigmondy in 1903. Con sult Zsigmondy,