up to the edge of the blade, gives the distance BC in terms of the half wave-length.
The distance, BD, was measured by the micrometer, and the number obtained by dividing BD by the distance between the consecutive bands in the neighbourhood of the edge (since the thin glasses are hardly at all bent so near their free ends) gives when added to N the number of half wave-lengths in DE.
After these measures had been made, a piece of flat glass was laid on the blade of the razor as in fig. (2) and the number of interference Fio. 2.
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bands which appeared between the edge and a line parallel to the edge, but distant BD from it, was counted; and this observation was repeated with the flat glass on the opposite side of the blade. The angle H P I, i.e., the angle between the two positions of the flat glass, was also measured. If the grinding of the razor was perfect and there was no rounding at the edge, no interference bands would be visible between the blade and the glass, but the two would be in contact up to the actual edge. Pig. (3) is a large scale cross-section of the blade in the neighbourhood of the edge. The thickness of the edge K L is
D 'E'—(D'P + GE') - (FK + LG)‘.
If we put N = number of bands between the clip A and the razor edge ; e = the distance between consecutive bands near the edge; j- = the number of bands between the flat glass and either side of the blade in a distance DB from the edge ; BD = a and HPI=
