. r >22 SUMMARY OK CURRENT RESEARCHES RELATING TO 



his results during the progress of an investigation of the dispersion of 

 sodium vapour. He had previously found that the path-difference under 

 which it is possible to obtain interference-fringes with helium (I) 3 ) light 

 can be more than doubled by the introduction of a small amount of 

 sodium vapour into the path of one of the interfering beams. This 

 development of fringes far out in the system by the dispersive action of 

 the vapour is accompanied by their complete disappearance at the centre 

 of the system, where the difference of path is zero. The author worked 

 with a narrow range of the spectrum symmetrical about the D lines. 

 This was obtained by opening the slit of the monochromatic illuminator, 

 bisecting it with a wire, and adjusting the prisms so that the region of 

 the I) lines was screened off by the wire. By means of a small screen 

 either of the two narrow portions of the spectrum bordering the D lines 

 could be screened off. The effect of the sodium vapour on the fringes 

 formed when the interferometer was illumined by either one or both of 

 the two portions of the spectrum could then be studied at leisure. It 

 was found that when a considerable amount of the vapour was present, 

 the apparent centre of the greenish-yellow fringe system was widely 

 separated from the centre of the orange-yellow system. When both 

 sorts of light were used at once, there was a periodic visibility in the 

 region in which the two systems overlapped. 



Crookes, Sib W. — Ultra-Violet Spectrum of Radium. 



[The author has, with some exceptionally pure material, 

 repeated the experiments of Runge, Demar<;ay, and 

 Exner and Haschek. His results differ materially from 

 theirs.] Proc. Boy. Soc, lxxii., No. 482 



pp. 295-304 (3 pis.). 



,. „ Ultra-Violet Spectrum of Gadolinium. 



[The author's experiments confirm those of Exner and 

 Haschek, but do not seem to support Urbain's view that 

 Gadolinium and Victorium are identical.] 



Op. cit., lxxiv. No. 504, pp. 420-2. 



Fabre, M. G. — Les perfeotionnements du Microscope. 



[The author gives an interesting resume of recent investigations on ultra- 

 microscopical bodies.] Mem. de VAcad. des Sci. de Toulouse, 



Dixieme Serie, iv. (1904) pp. 314-20. 



Hag a, H.— Ein Vorlesungsversuch fur die Bestimmung der Wellenlange des 

 Lichtes. Zeits. /. Vnterricht., xvii. (1904) p. 288. 



Marpmann, G. — Ueber ultramikroskopisches Sehen. 



[The author reviews our present knowledge of operating with ultra-violet rays.] 

 Zeits. f. ang. Mikr. u. Klinische Chemie, xi. (April 1905) pp. 1-7 



Merlin, A. A. C. E.—Amphipleura pellucida (Resolution of). 



English Mechanic, lxxix. (1904) p. 284. 



S o h i m m e l i' e n n i,n g, v o N deb O y e, V.— Zur Theorie du Doppelbrechung. 



Teil i. (Brunn, 1903) 29 pp. 



Schuster, A.— Introduction to Theory of Optics. 



London (E. Arnold), 1904, 356 pp. 



Stone y, Johnstone, G— How to Exhibit in Optical Instruments the Resolution 

 of Light into its component undulations of Flat Wavelets, and how to employ 

 this resolution as our guide in making and in interpreting experiments. 



Rep. Brit. Assoc Southport, 1903(1904) p. 568. 



