GEOMETRICAL MEASUREMENTS 171 



on the reflected image of the aperture T (the micrometer set at A' 

 being removed together with O^. Once eye accommodation is achieved 

 in the focal plane of the objective O^, the field is perceived brightly 

 illuminated and bounded by the objective's surface. If the plane 

 tangent to A at the surface S is perpendicular to the microscope's 

 optical axis, it is evinced that the tip of the reticle R is in aUgnment 

 with its image R (Fig. 5.7): the reason being that the rays, issuing 



Fig. 5.7. Field aspect if the surface S is normal to the"'microscope'"axis. 



from R, are reflected on the small and virtually plane portion of S 

 which is illuminated (image of the aperture T). They are reflected 

 back symmetrically and show a reversed image of the reticle R in 

 the focal plane of Oi. If the surface is not perpendicular to the mi- 

 croscope's optical axis at A, the reflected luminous cone rotates about A, 

 the imaged reticle R' shifting at the same time (Fig. 5.8). Then, adding 

 suitable elements to the microscope, enables one to carry out measure- 

 ments of flat-surface angles. It should be pointed out that with re- 



FlG. 5.8. Field aspect if the surface S is not normal to the microscope axis. 



ference to the reticle R and its image R' , all rays are reflected in a small 

 area surrounding A. From the diff'raction standpoint, the process 

 taking place for R and R' is as if the objective Oi had the same dia- 

 meter as the imaged aperture T sit A. The smaller the image, the 

 poorer the accuracy. Using an area of 5 // on a 0-7 mm diameter 

 cylinder (such as a needle) the direction of the perpendicular can be 

 determined within 15". If the area has a 1 ^ diameter, accuracy is 

 within 40". 



