MEASURING DRY MASSES WITH INTERFERENCE MICROSCOPES 215 



The problem is solved by changing over to white light. If the object 

 is so small that only a portion of the fringe is detected in its image, 

 the white light corresponding to zero path-difference (A = 0) is brought 

 on to the object whereupon the location of the white fringe in the 

 field surrounding the object is determined. The distance from the 

 small object itself to the white fringe of the field (outside the object) 

 is shown as monochromatic inter-fringe spaces. Such distance con- 

 stitutes the path difference. If the object is sufficiently extended as 

 to exhibit at least one fringe, as in Fig. 6.5, merely observing the white 

 fringe, within and outside the object, evinces the path difference which 

 is the inter-fringe distance reckoned as monochromatic inter-fringe 

 spaces. 



There remains to determine whether the object y4 is a prominence 

 or a hollow. Once more, white-light observation gives the clue to 

 the problem. A known object, e.g. a prominence is examined and 

 the position of the white fringe in the field (outside the object) is noted 

 when, concomitantly, this fringe is within the object. The object being 

 in the centre of the field, for instance, it is found that the white fringe 

 (outside the object) is to the left of the field. If the same arrangement 

 is observed with the unknown object then, a prominence is present. 

 Conversely, if the white fringe (outside the object) is to the right of 

 the field, a hollow is involved. 



Measuring a thickness with a polarizing interference microscope 



Measuring a thickness is inferred from the preliminary measurement 

 of a path difference. The methods shown in Chapter YII, §§ 1 and 2, 

 are applicable: the only change required is to substitute the diagram- 

 matic object shown in Fig. 6.1 by the one in Fig. 8.2. The wave re- 



T 



Fig. 8.5. The object reflected wave. 



fleeted by the object, then, has the shape shown in Fig. 8.5 (where 

 a hollow is involved). If e is the thickness of the detail examined, 

 owing to the reflection, the path difference is ^ = 2e. 



Let us consider the microscope shown in Fig, 4.30. The object- 

 reflected wave, having the shape shown in Fig. 8.5, penetrates into 

 the objective O^ and passes through the Wollaston at /. This wave 



15 



