MICKOTOMY 



H 



(a) 



(b) 



(d) 



(c) 



Fig. 2. Interferograms of different facet surfaces, (a) honed; (b) stropped; (c) lapped against cast- 

 iron; (d) lapped against glass. (Hg-light, 300X). The interference fringes reveal irregularities in the 

 surface. The distance between two fringes represents a profile depth of 0.27 fi. 



ployed must possess the horizontal resolvmg 

 power of the light microscope, as well as 

 the vertical resolving power required for 

 thickness determination. In Fig. 3 is shown 

 an apparatus for thickness determination 

 which meets the.se demands. It consists of 

 a commercial microscope for incident light. 

 To increase the focusing accuracy, a thin 

 metal wire has been moimted in the plane 

 of the field iris. By means of a special ar- 

 rangement with off-central incidence of the 

 light, this wire produces a shadow in the 

 object plane which moves back and forth 

 across the field of view during focusing and 

 defocusing. The line of symmetry for this 

 movement is indicated with a hair cross in 

 the eyepiece, and it is therefore possible to 



reproduce the focusing with great accuracy. 

 The vertical movement of the object i\-e dur- 

 ing focusing is registered by means of a 

 mechanical measuring device. In this way 

 it is possible to determine the thickness of a 

 section by focusing first on the upper surface 

 of the slide, and then on the upper surface 

 of the section. By means of a special con- 

 struction of the microscope stage, the slide 

 may be moved in the horizontal plane with 

 very small vertical deviations. If plane 

 parallel slides are used, it is possible to per- 

 form topographical analysis of sections. Un- 

 der ideal conditions the instrument permits 

 measurements with an accuracy of 0.1 n. 



Thickness Varialions in Sections. For 

 anybody planning to use sectioned material 



387 



