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 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



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[SIXTH SERIKfl^] 



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XII. On Methods for detecting small OpticaT^itgmrdations, 

 and on the Theory of FoucauW s Test. By Lord Rayleigh, 

 O.M., F.R.S.* 



AS was, I think, first emphasized by Foucault, the 

 standard of accuracy necessary in optical surfaces 

 is a certain fraction of the wave-lenth (\) of the light 

 employed. For glass surfaces refracting at nearly per- 

 pendicular incidence the error of linear retardation is about 

 the half of that of the surface ; but in the case of perpen- 

 dicular reflexion the error of retardation is the double of that 

 of the surface. The admissible error of retardation varies 

 according to circumstances. In the case of lenses and 

 mirrors affected with " spherical aberration," an error of ^X 

 begins to influence the illumination at the geometrical focus, 

 and so to deteriorate the image. For many purposes an 

 error less than this is without importance. The subject is 

 discussed in former papers f. 



But for other purposes, especially when measurements are 

 in question, a higher standard must be insisted on. It is 

 well known that the parts of the surfaces actually utilized 

 in interferometers, such as those of Michelson and of Fabry 

 and Perot, should be accurate to j 1 ^ \ to ^q\ and that a still 

 higher degree of accuracy would be advantageous. Even 



* Communicated bv the Author. 



t Phil. Mag. vol. viii. pp. 403, 477 (1879) ; ' Scientific Papers/ vol. i. 

 p. 414, §§ 3, 4. 



Phil. Mag. S. 6. Vol. 33. No. 194. Feb. 1917. M 



