614 SCIENTIFIC RECORD FOR 1885. 



in perpendicular planes. If now the field is viewed through a Nicol 

 prism, then according to the position of its principal section would the 

 one half at one time and the other half at another time be invisible. 

 So that if the two entering beams of light have different inteusities 

 their spectra may be equalized in brightness by rotating the Nicol; aud 

 from the degree of rotation the relative intensities be inferred. {Nature^ 

 June, 1885, xxxii, 191.) 



Balmer, in calculating the wave lengths of the hydrogen lines as given 

 by Angstrom, has found a relation between these lines expressed by the 



formula h=—- — jC, when has the value 3,645"6 millionths of a 

 vr — 4 



millimeter. In place of m let there be put in turn the numbers 3, 4, 5, 6; 

 and the values for Angstrom's wave lengths of the four visible hydrogen 

 lines are obtained. If for w, the values 7, 8, &c., up to 16 are used, 

 values for hydrogen lines are obtained which correspond very well with 

 the wave lengths of the lines which Dr. Huggins had found in the ultra- 

 violet spectrum of the white stars and had recognized as the invisible 

 hydrogen lines. This relation between the hydrogen lines had now re- 

 ceived an increased significance from an investigation by Cornu, in 

 which he had found a i:)erfectly determinate proportionality between the 

 lines of the ultra-violet spectrum of aluminum aud of thallium, and the 

 ultra-violet hydrogen lines. Like the hydrogen lines, the j)airs of lines 

 of the two metals referred to advanced much nearer to one another and 

 became mnch paler the more one approached the more refrangible 

 end of the spectrum; and if any line of the aluminum or the thallium 

 spectrum was made to coincide with the corresponding line of the hydro- 

 gen spectrum then did all the remaining lines coincide. This relation 

 obtained both for the first and for the second components of the pairs 

 of lines in the metallic spectra. (Nature, July, 1885, xxxii, 312.) 



Crookes has communicated io the Eoyal Society a paper on radiant 

 matter spectroscopy, in which he gives the results of further re- 

 searches on the prismatic analysis of the light emitted by the cerium 

 group of earths when made to phosphoresce by the impact of radiant 

 matter in high vacua. With regard to the double orange band observed 

 in 1881, he details at length the extraordinary difficulties overcome in 

 localizing it. "After six months' work I obtained the earth didymia in 

 a state which most chemists would call absolutely pure, for it contained 

 probably not more than 1 part of impurity in 500,000 parts of didymia. 

 But this 1 part in 500,000 profoundly altered the character of didymia 

 from a radiant matter sx)ectroscopic point of view, and the presence of 

 this very minute quantity of interfering impurity entailed another six 

 months' extra labor to eliminate these traces and to ascertain the re- 

 action of didymia pure and simple." Gradually the matter was nar- 

 rowed down and the orange band was finally traced to samarium. Pure 

 samarium sulphate alone gives a very feeble phosphorescent spectrum. 

 But when the samaria is mixed with lime the spectrum is, if anything, 



