Intelligence and Miscellaneous Articles. 367 



rays of light, which have probably their origin in the fact that the 

 dimensioDS of the reflecting ray are too small in comparison with 

 the wave-length of the ray. — Wiener Berichte, Feb. 19, 1891. 



ON THE OPTICAL PROPERTIES OF a-MONOBROMONAPHTHALINE. 

 BY B. WALTER. 



It frequently occurs in researches on fluorescence, phosphor- 

 ence, &c, that the well-known powerfully dispersive media prisms 

 of flint glass and of carbon bisulphide cannot be used, because they 

 completely extinguish the ultra-violet portion of the spectrum. It 

 is therefore of interest to direct attention to the remarkable pro- 

 perty of a-monobromonaphth aline, which, in addition to its great 

 dispersive power, has also a remarkably complete transparency for 

 ultra-violet rays. For instance, I caused the spectrum produced by 

 a hollow prism filled with this material to fall on a trough con- 

 taining solution of aesculine, and I could observe the solar spectrum 

 in the ultra-violet beyond N. That the absorption which occurred 

 from this point was not caused by the monobromonaphthaline, but 

 by the various glasses which I had to use, follows from the fact 

 that the spectrum was the same whether I allowed the rays to pass 

 near the base or near the apex of the prism. By comparing 

 photographs of diffraction -spectra with those of monobromo- 

 naphthaline, I am convinced that the absorption of ordinary crown 

 glass extends just into those regions. 



If, in addition to these properties of great dispersion and trans- 

 missibility for ultra-violet, we remember that its boiling-point is at 

 277° C, that it has not the objectionable smell of carbon bisulphide, 

 and that its refractive index for each degree Centigrade only 

 increases by O0 3 48 instead of O0 3 80 with carbon bisulphide, we 

 shall see that optical science has a very valuable acquisition in this 

 substance. It is said that it alters somewhat with time ; but this 

 cannot be very considerable, as the liquid in my possession has 

 still the same properties without alteration as at first. My deter- 

 minations also of the refractive indices agree very well with the 

 individual statements of the older observers (Fock and Pulfrich); 

 but they differ from those of Dufet {Journal de Physique [2] iv. 

 p. 415, 1885), although both preparations proceed from the same 

 source. 



Refractive indices of a-monobromonaphthaline at 20° C. : — 



A. a. B. C. D. E. 



1-64051 1-64367 1-64638 1-64948 1-65820 1-67049 

 F. H y . G. m. H. 



1-68195 1-70410 1-70595 1-71855 1-72893. 



The letters refer generally to Fraunhofer's lines, excepting H y 

 and H<5, which denote hydrogen-lines. The refractive index of the 

 D line decreases 0*0 a 48 for 1°. The density of my monobromo- 



