ENGINEERING MICROSCOPES 



ENGINEERING MICROSCOPES 



diaphragm is always available for control of good centration of the optics, although eye- 



the illuminated N.A. piece centration is still much less critical 



Nosepieces. The rotating nosepiece, used than objective centration. 

 to interchange objectives, is a remarkably 



precise mechanism. Repeatability of cen- James K. ±5enford 



tering is commonly held to within 0.00005", 

 a value attained chiefly by a precisely fitted 

 bearing and a sturdy and well designed click- 

 stop. The bearing may be either a central There are many applications of standard 



cone bearing, or a peripheral ball bearing, and specialized microscopes to the field of 



The click stop may be either a V-shaped engineering in production and testing of 



spring clicking onto a peripheral ball stop, or manufactured articles. A few of the impor- 



a spring-loaded rotatable ball clicking into a tant examples are cited to show^ the great 



notch on the rim of the nosepiece. In either importance of microscopy in engineering, 

 case, the spring is a very flat leaf spring, Surface Examination. Perhaps the most 



having good rigidity in its wide dimension direct application is the examination of sur- 



to hold centering precisely, but yielding faces particularly of metallic specimens, for 



smoothly in a radial direction to give just the detection of flaws and cracks and for 



the correct force to click the stop mecha- the distinction of surface finish. When a 



nism home. crack is of the order of 0.0001 in. in width, 



Objectives. Objectives are almost uni- or when a sm-face structure is equivalent to 

 versally threaded in conformance with the a series of rulings of the order of 10,000 to 

 Royal Microscopical Society standard, so the inch or less, a microscope is required. A 

 that objectives of one manufacturer may be conventional microscope discloses the extent 

 interchanged with those of another in a or width of markings in a plane view, but 

 nosepiece. The virtue of this standardization depth measurement requires a special type 

 is somewhat lessened by the fact that the of microscope, the Schmaltz Profile Micro- 

 objectives are not standardized for shoulder scope discussed in a later section. Require- 

 to specimen distance, hence the convenience ments are adequate illumination of the sam- 

 of parfocality is lost when intermixed objec- pie, preferably from a vertical illuminator; 

 tives are used on a multiple nosepiece. and adequate resolving power (R.P.) of the 



The lens elements are commonly bur- optical system (see Optical Theory of Light 



nished into individual cells, which in turn fit Microscope). This R.P. or the minimum dis- 



into a common bore in the objective barrel, tance between tw^o points which appear as 



The fit of cells into the bore and the centra- two separate images is given by R.P. = 



tion of lens elements in their cells is the most 0.61X/NA, when X is the w^ave length of light 



difficult part of making a good objective, the used and NA is the numerical aperture, 



tolerances being unbehevably tight. w^hich is a measure of the maximum cone of 



Eyepieces are not greatly different in the light which the microscope objective can 

 modern microscope than in the microscope of take in and refract to the observer's eye. For 

 50 years ago. The simple 2-lens Huygens the majority of cases of examination of ma- 

 construction has stood the test of time and chined surfaces the microscope is suitably 

 is still the most widely used eyepiece in mi- fitted with a % m. objective of NA 0.28 

 croscopy. Probably the most significant and an eyepiece of powder 20 X. The labora- 

 change in eyepieces is the requirement for tory microscope is suitable when specimens 

 better centering today because of the in- are not too large, but in other cases special 

 creasing percentage of binocular models. To workshop microscopes of rugged design are 

 get proper results binocularly requires quite available. 



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