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THE TESTING OF THE

Online Encyclopedia
Originally appearing in Volume V18, Page 407 of the 1911 Encyclopedia Britannica.
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THE TESTING OF THE MICROSCOPE The excellence of a microscope objective depends on its definition and its resolving power. The definition is better according as the chromatic and spherical aberrations are removed ; there always remains in even the best constructions some slight aberration. In consequence of these residual aberrations, every object-point is not reproduced in an ideal image-point, but as a small circle of aberration. These circles will be objection- able when the smallest details are examined. The size of these circles depends, in the case of equal tube lengths, only on the type of the objective, and not on the focal length, exact execution being assumed. Object details will only be well seen if the aberration circles are small in comparison. The size of these details in the image depends only on the magnification of the objective, M =A/fi', and can by appropriate choice of the focal length of the objective be brought to the right value. In the case of a suitable ocular magnification, the details will be well seen, while the aberration circles remain invisible. It is therefore possible to judge the excel- lence of the focusing of objectives on the over-magnification, which they permit. DD=diaphragm. E. Abbe, through the so-called delicate K1K2=condenser. ray transmission, suggested a way by which L=front lens of the the quality of the images of objectives can be objective. observed. The ray transmission, shown in The lower figure fig. 55, is obtained by means of a stop shows the plan of of the form shown in the lower figure the transmission. and placed under the condenser in the plane of the iris diaphragm. The entrance pupil is in this way reduced on two small separate fields, which nevertheless contain rays of all zones. It is necessary that the outside edge of the diaphragmcoincides with the edge of the entrance pupil. This can be attained by drawing the iris diaphragm so far as to form the entrance pupil. The double diaphragm is then in such a position that the edge of the outer diaphragm coincides with the edge of the iris diaphragm. The object employed must have distinct boundaries. Abbe's test plate consists of an object carrier on which six cover glasses of exactly determined thickness (between 0.09 mm. and 0.24 mm.) are cemented. The cover glasses are silvered on their under surfaces, and in the silvering fine lines are drawn; these lines form the test object. This plate admits at the same time of a correct determination of the thickness of the cover glass, for which the best correction exists. So long as the object is not sharply focused two separate dispersion figures will be seen. The defects of the objective are revealed, e.g. two adjacent sharp images are formed, which become indistinct if they coincide, or one pencil produces a distinct, the other an indistinct image, or that the images are surrounded with coloured rings. Owing to the curvature of the image, all parts of the object are not seen distinctly at one and the same time. The resolving power of an objective depends on its numerical aperture. The numerical aperture can be determined in two ways. A diaphragm with a very narrow hole is placed on the stage, and the microscope sharply focused on the edges of the hole. The illuminating mirror is turned aside and a graduated scale is laid on the foot of the microscope. Strong systems produce in the proximity of their back focal plane an image of the scale, which can be inspected with a weak auxiliary microscope, and the length of the visible part of the graduation determined. The ratio of half the length of the visible piece of the scale to its distance from the diaphragm on the stage gives the tangent of half the angular aperture. The sine of this angle is the numerical aperture for dry lenses. With weak systems no auxiliary microscope is necessary, the eye-piece being removed and the scale viewed directly in the tube. E. Abbe constructed a simple instrument for the determination of the aperture, termed the apertometer (fig. 56). A semi-circular A°moo° .n~/~b//1e; AA 0 d ~'- C.Zeiss Abbe's Jena Apertometer pi At. View from above Side view glass plate bears two scales, over which two black thin metal plates bent back at right angles may be moved. A little hole in the silvered plate a marks the centre of this circle. Through this hole the points of the metal plates b can be observed by total reflection on the surface c. The apertometer is laid on the stage, so that the hole lies in the axis of the microscope, and the hole is sharply focused. The eyepiece being removed the image of the metal plates b produced by the objective is seen. In order to ensure for the eye a central position, there is fixed on the upper end of the tube in place of the eyepiece a disk of pasteboard or metal with an axial hole. The metal plates b are then moved till the points just cut off the edge of the field to be surveyed. The angular or numerical aperture can then be read off. With strong systems the vanishing of the points is observed with an auxiliary microscope, formed by means of the inner tube. In immersion systems the immersion liquid is placed between the front lens and apertometer. If the numerical aperture be known the resolving power is easily found. The resolving power can also be determined by using different fine test objects. Norbert's test plates, which bear graduated groups of extremely fine and narrow divisions are very useful, while the tests of Amphipleura pellucida and Surirella gemma are often employed. The magnification of a microscope is determined from the focal lengths of the two optical systems and the optical tube length, for N =250 0/fl'f2. To determine the optical tube length 0, it is necessary to know the position of the focal planes of the objective and of the ocular. If one focuses an auxiliary microscope, carried in the inner tube, on the image situated in the back focal plane of the objective of a distant object, and then on the dust particles lying on a slide pressed against the end of the outer tube, the displacement of the auxiliary microscope gives the distance of the back focal plane of the objective from the end of the outer tube. To determine the position of the anterior focal plane of the eyepiece, the eyepiece is placed on the stage with the eye-lens downwards. An auxiliary microscope is now focused first on the image of a distant object and then on the plane of the edge of the setting. This plane can be marked by a small piece of paper. This gives the distance of the anterior focal plane of the eyepiece from the bottom edge of the setting of the eyepiece and consequently also of the edge of the eye-piece carried by the upper end of the tube. These measurements determine the optical tube length A. There are many methods for determining the focal length of the objective. The objective to be examined is placed on the stage, and in the manner just shown, the distance of the focal plane from the edge of the fittings or to the surface plane of the front lens is deter-mined. Any plane object a few yards distant can be used. If the object can be seen by using the mirror, the plane mirror must be used; then the actual size of the object and of the image produced by the objective is measured (of the image by a micrometer ocular). The distance of the object from the nearer focus of the objective is next determined. This distance is composed of the distance of the object from the centre of the plane mirror, and of the distance of the focus of the objective on the stage plate from the centre of the plane mirror. Let the size of the object be y, the size of the image y' the distance of. the object from the focus x, then y/y' =x/fi from which fi can be calculated (see LENS). The same method can be used to determine the focal length of the eyepiece. These are the dimensions necessary for determining the magnification of the micro-scope, viz. the optical length of the tube A, the focal lengths of the objective fi', and of the eyepiece f2. The focal length of an objective can be more simply determined by placing do objective micrometer on the stage and reproducing on a screen some yards away by the objective which is to be examined. If the size of the image of a known interval of the objective micro-meter is determined by an ordinary scale, and the distance of the image from the focal plane of the objective belonging to it is measured, then the focal length can be calculated from the ratio y/y' =fi'/xi', in which y is the size of the object, y' that of the image, and x1' the distance of the image from the focal plane belonging to it. Besides this indirect method of determining the magnification there is also a direct one, in which it is not necessary to first measure f2 or A. If a drawing prism is used above the eyepiece, and an objective micrometer is inserted, then if a scale is laid on the drawing board which is 25 cm. distant from the exit pupil, one or more intervals of the objective micrometer can be seen projected on the scale lying on the board. The comparison of the two scales gives directly the magnification. The course of the light within the drawing prism must be taken into account when determining the distance of the scale from the exit pupil. Although this method does not give very accurate results, it is more convenient and simple than the indirect method.
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