Retardations, and on the Theory of Foucault 3 s Test, 163 



Another method by which moderately small retardations 

 <3an be made evident is that introduced by Foucault * for the 

 figuring of optical surfaces. According to geometrical 

 optics rays issuing from a point can be focussed at another 

 point, if the optical appliances are perfect. An eye situated 

 just behind the focus observes an even field of illumi- 

 nation ; but if a screen with a sharp edge is gradually 

 advanced in the focal plane, all light is gradually cut off, 

 and the entire field becomes dark simultaneously. At this 

 moment any irregularity in the optical surfaces, by which 

 rays are diverted from their proper course so as to escape 

 the screening, becomes luminous ; and Foucault explained 

 how the appearances are to be interpreted and information 

 gained as to the kind of correction necessary. He does not 

 appear to have employed the method to observe irregularities 

 arising otherwise than in optical surfaces, but H. Draper, in 

 his memoir of 1864 on the Construction of a Spherical Glass 

 Telescope t, gives a picture of the disturbances due to the 

 heating action of the hand held near the telescope mirror. 

 Topler's work dates from the same year, and in subsequent 

 publications ± he made many interesting applications, such 

 as to sonorous waves in air originating in electric sparks, 

 and further developed the technique. His most important 

 improvements were perhaps the introduction of a larger 

 source of light bounded by a straight edge parallel to that of 

 the screen at the observing end, and of a small telescope 

 to assist the eye. TTorthy of notice is a recent application 

 by R. Cheshire § to determine with considerable precision 

 for practical purposes the refractive index of irregular glass 

 fragments. TVhen the fragment is surrounded by liquid || of 

 slightly different index contained in a suitable tank, it appears 

 luminous as an irregularity, but by adjusting the composition 

 of the liquid it may be made to disappear. The indices are 

 then equal, and that of the liquid maybe determined by more 

 usual methods. 



We have seen that according to geometrical optics (\ = 0) 



* Ann. de I'Observ. de Paris, t. v. Collected Memoirs. Paris, 1878. 



t ' Smithsonian Contribution to Knowledge/ Jan. 1864. 



1 Pogg. Ann. Bd. cxxviii. p. 126 (1866) ; Bd. cxxxi. pp. 33, 180 

 (1867). 



§ Phil. Mag. vol. xxxii. p. 409 (1916). 



|| The liquid employed was a solution of mercuric iodide, and is 

 spoken of as Thoulet's solution. Liveing (Camb. Phil. Proc. vol. iii. 

 p. 258, 1879), who made determinations of the dispersive power, refers 

 to Sonstadt (Chem. Xews, vol. xxix. p. 128, 1874). I do not know the 

 date of Thoulet's use of the solution, but suspect that it was subsequent 

 to Sonstadt's. 



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