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<p>By slight modifications of the Fabry and Perot procedure of employing interference fringes the author claims to have instituted a more sensitive experimental routine, with consequent improvement in the accuracy of determination of secondary standard wave-lengths. The method is characterised by three chief features: (1) The use of reflection fringes converged by a very short focus lens (20 mm.), thus enabling the disappearances of the bright central spot of the interference rings to be accurately counted; (2) the use of the double increment in the distance between the two mirrors instead of the double distance as the difference of path; (3) the use of the method of flexure for the measurement of the fractional part of the order of interf
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<br>Subjects: measurements and use of instruments; spectra</div>
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