374 Prof. Carslaw on the Use of Contour 



Our only means o£ obtaining more powerful instruments 

 seems to lie. therefore, in the further development of the 

 grating-spectroscope. The maximum resolving-powers that 

 have thus far been attained with the latter class of instruments 

 are somewhat less than 400,000 units *, about the maximum 

 which, as we have now shown, is attainable with prismatic 

 spectroscopes in the visible region of the spectrum. As the 

 writer has already shown f instruments having at least three 

 times this resolving-power are not only exceedingly desirable, 

 but absolutely necessary to the successful investigation of 

 certain spectral phenomena. We must therefore aim to 

 ultimately produce gratings at least 40 cms. diameter. A 

 machine designed to rule gratings of this size was begun by the 

 writer seven years a^o (1896). but owing to the many inter- 

 ruptions to which reference was made at the beginning of the 

 article, work on it was only recently completed ±. The con- 

 struction of a machine of about the same size was begun in 

 1899 by Professors Michelson and Stratton at the University 

 of Chicago ; but work on this has also been much interrupted 

 and delayed by the resignation of Professor Stratton to accept 

 the directorship of the new National Bureau of Standards at 

 Washington. More recently still Jewell, at Johns Hopkins, 

 has been remodellmo- one of the Rowland dividmo-eno-mes to 

 rule 10 in. (25 cm.) gratings. 



It is to be hoped, therefore, that through the united efforts 

 of all of those now working on this problem, gratings of a 

 considerably larger size than we now have may soon become 

 ii commercial possibility. 



Allegheny Observatory, November, 1902. 



XXXV. The Use of Contour- Inter/ration in the Problem of 

 Diffraction by a Wedge of any Angle. By H. S. Caesiaw, 

 Professor of Pure and Applied Mathematics in the University 

 of Sydney, A .S. W.§ 



IX the Appendix on Diffraction to his Adams Prize Essay || 

 Macdonald gives a discussion of the two-dimensional 



* See paper on " Resolving- Power of Telescopes and Spectroscopes 

 for Lines of Finite W idth," Phil. Mag. vol. xliii. p. .120, Mav 1897. 



t Loc. cit. The large solar grating-spectroscopes of the new 

 Allegheny Observatory are designed for gratings of 25 cms. and 40 cms. 

 aperture Report of Director, 1900. 



\ See Report of the Director, Allegheny Observatory, 1900, p. 22. 

 The final setting up of the new engine has been still further delayed by 

 the delay in the completion of the new Observatory and Laboratories. 



§ Communicated by the Author. 



|| 'Electric Waves,"' by H. M. Macdonald, Camb. 1902. 



