CHAPTER V 



Geometrical Measurenieiits 



1. SETTINGS IN MICROSCOPY 



Microscope measurements are based on two types of settings: 

 transverse and axial. The former entails no alteration in sharpness. 

 Let us consider, for instance, a small object whose tips are properly 

 focused. In microscopes comprising a micrometer eyepiece, the spider- 

 line is shifted so as to align it seriatim with the imaged edges of the 

 object. As the spider-line shift is shown by a micrometer screw, the 

 width of the observed object can be readily measured. These operations 

 entail two transverse settings. A transverse setting is made every 

 time the spider-line of the eyepiece micrometer is brought in alignment 

 with the imaged point of an object without altering the focusing and 

 the spider-line is shifted transversely. The axial setting is a sharpness 

 setting. For instance, it is required to locate the position in depth 

 of a given point of the image: to do this, the fine adjustment is actuated 

 until the point considered is sharp concurrently with the eyepiece 

 reticle. Shifting the microscope axially brings the imaged, considered 

 point, in coincidence with the eyepiece reticle. The object-point lies 

 in a definite position in depth with respect to the microscope and 

 is so located by an axial setting. 



2. SETTING ACCURACY 



When the microscope is fitted with a 05 mm diameter exit pupil 

 (optimum magnification) setting accuracy is practically restricted by 

 the diffraction laws (cf. Chapter I, § 8). Let us first investigate this case. 



Transverse setting accuracy 



Let A be the focused object (Fig. 5.1). To eff'ect a transverse 

 setting, the spider-fine R of the reticle is shifted in a front plane so 

 as to be in coincidence with the image A' of A. Now, let us assume 



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