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Originally appearing in Volume V18, Page 403 of the 1911 Encyclopedia Britannica.
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ILLUMINATING SYSTEMS Most microscopic observations are made with transmitted light; an illuminating arrangement is therefore necessary, and as the plane of the object is nearly always horizontal or only slightly inclined, the illuminating rays must be directed along the optical axis of the microscope. To fully utilize the aperture of the system all dispersing rays in the object-space of the objective must be retained in the image-space of the illuminating system. When this occurs the greatest brightness will be obtained if the corresponding diaphragms of the two systems coincide; i.e. the field-diaphragm on the image-side of the observing system with object-side of the illuminating system, and the exit pupil of the illuminating system with the entrance-pupil of the objective. For slight magnifications a revolving plane mirror fixed below the object for altering the direction of the rays suffices. For this mirror to illuminate all the points of the objective so that the rays fill up the objective, it must not be too small, and should be as near as possible to the stage plate, and the source of light must be con- siderably extended (fig. 40). Diffused daylight is very suitable. If the aperture of the objective is increased, the diameter of the illuminating surface must also be increased so that the system is quite filled up, from which it follows that this method of illuminating soon fails. The possibilities of illuminating with a concave mirror seem a little more favourable. As a rule a concave mirror of similar aperture is fitted on the other side of the plane mirror. With the con-cave mirror an image of the source of light can be thrown upon the object. The distance of the concave mirror from the stage plate is about equal to its focal length. This is also the most suitable distance when diffused daylight is used, but it is too short with artificial light; the distance between the stage plate and the mirror should then be increased, so that an image of the source of light can be thrown upon the object. It is simpler to place an illuminating lens in front of the source of light so that the source falls approximately at the front focus of this lens and consequently is represented at infinity through the illuminating lens. By a correct choice of the focal length of the illuminating lens in relation to the focal length of the mirror, it is possible to choose the size of the image of the source of light so that the whole object-field is uniformly lighted. Too much light is useless for observing delicately coloured or colourless preparations, whose parts only become visible as a result of slight differences of diffraction. Then it is necessary to use powerfully concentrated cones of light. The apparatus must be such that the apertures of the illuminating rays can easily be altered, e.g. by inserting diaphragms in the course of the rays of the illuminating cone below the stage plate (fig. 4o, PP). This concentration is most easily produced by sliding or revolving diaphragms. A series of holes of different sizes perforate a revolving disk below the stage plate at an equal radial distance from the axis of the disk, so that the holes can be brought under the preparation in turn, the centre of the diaphragms always being a continuation of the optical axis of the microscope.sometimes fitted in a slide, so that it is possible to move the diaphragm sideways and give oblique illumination (see below). With very powerful objectives these methods are insufficient; and a condenser is fitted below the stage plate. As a rule an iris diaphragm, which can be moved sideways, is now fitted below this condenser; below is the mirror which can be moved in all directions. The Abbe apparatus consists of a condenser, movable iris diaphragm, and mirror (fig. 42). The whole apparatus can be focused by a rack Condenser (Zeiss). and the button s. The iris diaphragm can be regulated by the lever p; it can also be turned to one side round the pivot z, so that the condenser k can be removed or changed. The correct direction can be given to the illuminating cone by the mirror m. It is often desirable to pass from direct to oblique lighting. The Abbe apparatus makes this'easy. The iris diaphragm i is pushed to the side by the rack and pinion t n. The chief cone of rays then enters obliquely into the objective, the angle between the direct cone of rays and the diffraction spectrum of the first order can then become as large again as with direct lighting, and still be taken up in the objective. Oblique lighting, however, can only be in an azimuth, so that the object must be turned in order that the details may be observed. Hence a condenser, for lighting with very oblique cones, must have about the same aperture as the objective, and therefore be of very wide aperture; they therefore closely resemble microscope objectives in construction. Especially powerful achromatic condensers are really only magnified microscope objectives, with the difference that they are not corrected for the thickness of the cover slip, but for the thickness of the glass on which the object is placed. For exceptionally accurate work microscope objectives are sometimes used as condenser systems. When using immersion objectives, an immersion condenser must also be used if rays of extreme obliquity are wanted, for, in consequence of the total reflections, rays can only come from the upper plane surface of the condenser, which have not a larger inclination to the axis than about 41 °, varying according to the refractive index of the glass. In order to let highly inclined rays pass out from the condenser, some immersion liquid must be placed between the upper surface of the condenser and the object slide. Condensers are for this reason also constructed with apertures up to 1.40. Vertical Illuminators.—Opaque objects can only be seen by reflected light. With low magnifying systems and a large free object distance, ordinary good daylight is sufficient. If the objects have a low reflecting power, or if a slightly higher magnification is needed, the lighting can be improved by optical system. To examine small opaque objects with a high magnification the Lieberkiihn mirror, so named after its inventor, was formerly much used. This was a concave mirror, pierced in the middle, fixed to the objective, and directed towards the object and with such a ',1114111111,4110w I Iti'o ri II~ dW!:':Illi I ,l I tion. Ml = plane-, M2 = curved-mirror. 0 = object; LI= front lens of microscope; PP = diaphragm. The so-called cylinder diaphragms (fig. 41) are used especially in German microscopes. A changeable diaphragm is placed at the upper end of a short tube which can be moved in a case below the stage in the direction of the optical axis. By bringing the diaphragm creased; if the diaphragm is removed farther from the object the cone of rays is diminished (cf. fig. 40). These diaphragms are focal length that rays parallel to the axis falling upon it were united exactly upon the object. In this case the object lay upon a stage plate, whose centre had so far been made opaque, so that the rays coming from the illuminating plane mirror could not reach the objec- tive direct, but only the rays passing the stage plate to the side of this blackened portion reached the Lieberkiihn mirror, and were used in lighting. The disadvantage of this method was that only small opaque objects could be examined. Much more easily manipulated is the parabolic side-illuminator invented by R. Beck, which can be conveniently fitted in and used for objectives with different focal lengths. It consists in half of a short focused parabolic mirror, which concentrates all the light coming from the one side on to the object. To examine objects with objectives of high power and low free object distance, the apparatus for side-illumination is not sufficient, and a so-called vertical illuminator is used. In Zeiss's form (fig. 43) a small prism p, which also revolves upon a horizontal axis, is placed as near as possible to the back lens of the objective. The edge which is the separating line of the horizontal and hypothenuse surfaces of the prism, lies approximately over the middle of the system, so that the rays entering through the opening in the side after having been reflected by the hypothenuse surface are concentrated through one half of the objective on to the object. When observing only the other half of the objective is used. The sources of light used should be arranged so that the objective throws an image of the light-source upon the object. It is best if the image of the light is not larger than the object examined, and to effect this, an illuminating lens with an iris dia- phragm is often placed between the source of light and the illuminator. By Illuminator (Zeiss). the iris diaphragm the size of the illuminating field can be controlled. The objects observed with the vertical illuminator must not have a glass cover if the dry system is employed, because the upper surface of the glass cover would send so much light back into the objective by reflection, that the image would be indistinct. It is, on the contrary, possible to examine covered objects with the vertical illuminator, if the immersion system be employed. Owing to the slight difference of illumination between the immersion liquid and the cover, the portion of light reflected on the cover is not noticeable. Dark Field Illumination.—As was seen when discussing the physical theory, the minute details of the object cause diffractions, and can only be examined if the objective can take up at least two consecutive diffraction spectra. These diffracting details become especially distinct if the direct lighting cone of rays, the spectrum of zero'order or the chief maximum, is not allowed to enter the objective and instead only two or more diffraction maxima are taken up; the details then appear bright on a dark background. In dark field illumination care has to be taken that no direct rays reach the objective, and hence a good dark field illumination can be produced if the condenser system has a larger aperture than the objective. If an Abbe lighting apparatus is used a dark field diaphragm (fig. 44) can be placed in the iris diaphragm case. The central diaphragm disk keeps away all the light which would otherwise fall directly into the objective, and the open zones send so many oblique rays through the object that they cannot all be taken up by the objective. FIG. 44. Exactly the same effect is reached when, as is shown in Fig. 45, a more powerful system D is used for a condenser, which has a blackened section on the back of the front lens of such a size that no light can enter the objective A. In this way it is only possible for diffracted rays to enter the objective. Apparatus for a good dark field illumination has received much attention, because in this way ultra-microscopical particles can be made visible. This depends on the good combination of the entering cones of rays, which should be as oblique as possible; this is most easily done by mirror condensers. A number of early inventions have been revived for this purpose. Wenham's paraboloid illuminator (fig. 46) is made entirely of glass, and is in the form of a paraboloid, having on the top a spherical hole, of such a curvature that all entering rays, r r' r", parallel to the axis, after their reflection on the surface of the paraboloid, traverse the spherical surface perpendicularly and unite in F, the centre of the sphere. A diaphragm s is placed in the middle of the spherical surface, and this keeps back the central rays. This diaphragm is sometimes fixed to a handle piercing the condenser, and which can be moved up and down, so that the aperture of the oblique entering cones of rays can be altered. Another form of the paraboloid condenser, also due to Wenham, has a plane surface on the upper side. Some immersion fluid must then be placed between the stage plate and the condenser in order to allow all the rays to pass out ; otherwise only those rays would be able to pass out which are403 close to the axis of the condenser in the inside of the condenser, and are smaller than the limiting angle of the total reflection. nla.u.n~mamm W PIl11OlO0l10RM MA "llll llltl %ift... , diaphragm in the objective. (Objective D can also be used as a condenser (Zeiss)) Th. Ross's " spot lens," invented in 1855, and J. W. Stephenson's catoptric illuminator (1879), may also be mentioned. A recent condenser of very high illuminating power is due to H. Siedentopf (fig. 47). It is a double mirror system, whose reflecting surfaces are a sphere a and a cardioid b. The combination of rays is also sufficient in practice if the cardioid surface is replaced by a spherical one. r" rr 11 1 Paraboloid Condenser. Cardioid Condenser. A supplementary spherical surface c is necessary for the completion of the condenser.
End of Article: ILLUMINATING
ILLUMINATI (Lat. illuminare)

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