Microscopy

MICROSCOPY, the art of using the microscope. There is little need for stressing the importance of the microscope in the investigation of many widely different problems. In medical, bio logical, geological, metallurgical and many other kinds of work, the value of the microscope is well understood, and the instrument is used constantly in routine work and in research. In other appli cations the value of the microscope is being increasingly realized, but its value might be appreciated even more widely if all users of the instrument should make a study of its principles, sufficient to enable them to realize the full implications of their observa tions. Such study would also prevent erroneous deductions from appearances seen in the image, appearances which might be due to faulty adjustment either of the instrument itself or of the illuminating apparatus. (See MICROSCOPE.) Methods of Illumination Commonly Used.—The most commonly used method of illumination is that in which the object is illuminated by transmitted light. This method is peculiarly liable to give images in which the contrast is seriously impaired due to "glare" effects if the cone of illumination is made sufficiently large for the aperture of the object-glass to be fully utilized. The importance of this method of illumination is not to be be littled, however. It gives information of much value and is convenient to employ ; it must, also, be the chief method of examining such objects as thin stained sections of animal and vegetable tissues. The information obtainable with any single type of illumination is necessarily incomplete, however, and should be supplemented where possible by examining the object under other conditions.

Among other methods of illumination ordinary darkground illumination, vertical illumination and side illumination are prob ably the most commonly used. The ways in which the micro scope and illuminating apparatus are adjusted to provide these various forms of illumination are fully dealt with in many treatises and textbooks on the microscope and microscopy (see also MICRO SCOPE). We must confine ourselves here to emphasizing the neces sity of mastering the methods of making these adjustments. The reader is referred to these textbooks, also, for information regard ing the appearances obtained in examining objects by these methods and by such other methods as are commonly used in medical, biological, geological and other types of work in which the technique has become almost standardised. We shall restrict ourselves to supplementing that information by giving (a) a few examples to illustrate the interpretation of some of the less well understood appearances obtained by the more usual methods of illumination, and (b) descriptions of some of the results obtain able by the use of novel or less commonly used technique, which serve to illustrate the possibilities of extended use of the micro scope for obtaining information as to the nature of the object being examined.

The Illuminant.

A powerful lamp with a small radiant will serve as the source of light for every kind of work to be done with the microscope. The use of a small radiant minimises the amount of light reflected down on to the top of the object from the sur faces of the individual lenses in the object-glass, or from the cover glass, when transmitted illumination is used, and so reduces the loss of contrast due to lens flare and cover-glass glare. If it is desired to use a larger area of illumination than that obtained by focussing an image of the radiant on to the object, this can be done by placing a bull's eye lens or a piece of ground glass, or both, in front of the lamp, and producing a focussed image of either of these on the object. A small radiant is not essential, however, and much useful work can be done with a simple paraffin lamp or with ordinary gas or electric lamps.

Recognition of Structure.

Although there is a limit to the resolving power of any object-glass, this does not mean that an object-glass cannot give indications of structure in objects having structures appreciably finer than the lens could possibly show resolved. Small particles have the power of scattering light which falls upon them. The intensity of the scattered light is greatest in directions at right-angles to the illuminating rays, and the per centage of incident light scattered is greater for blue light than for light of longer wave-lengths. The light directly transmitted by an object which contains discrete minute particles is thus deficient in blue and appears as of a yellow or brown colour, while the light seen by looking sideways at the object appears blue. If the par ticles are regularly arranged in the object there is, in addition to the scattered light, a certain amount of light which leaves the object along definite directions inclined at definite angles to the main beam. This light is diffracted light. The diffracted beams obtained from white light are seen as coloured bands, because the vibrations corresponding to the different colours which make up "white" are diffracted along different directions. If the whole of the light in any one diffracted beam is recombined, the result ing "colour" is white. If, however, only part of the light in a dif fracted beam is recombined, the colour obtained depends on the colours present in the used part of the diffracted beam.