( 544 ) 



other and to the former basepoints have not a particular position. 



With the aid of this remark we can easily find the following 

 theorems, with which we conclude : 



"Both cusps of which the fourfold point of the curve c la coin- 

 ciding with a basepoint A\ seems to consist and the two cusps of 

 the curves of the system showing in this point a cusp, coincide in 

 cuspidal tangents, but they turn their points to opposite sides." 



"If the three basepoints A\, A'.,, A\ lie on a right line /, the locus 

 proper of the cusps reduces itself to a curve c 9 touching the line / 

 in A\, A\, A\. If the three remaining basepoints exist then the 

 points of intersection of / with the sides of the triangle having those 

 basepoints as vertices are points of c 9 ". 



The last case answers to that of a surface S* with a double point ; 

 the parabolic curve having in this doublepoint a threefold point, 

 because I separates itself three times from c l \ is as has been found 

 above already a twisted curve of order nine. 



Physics. - --An investigation of some ultra-red metallic spectra." 

 By W. J. H. Moll. (Communicated by Prof. W. H. Julius). 



(Communicated in the meeling of December 29, 19U6). 



Among the spectra of known elements those of the alkali-metals, 

 by their relatively simple structure, lend themselves particularly well 

 to an investigation of their ultra-red parts. Many observers have 

 consequently sought for emission lines of these metals in this region. 



For the first part of the ultra-red spectrum the photographic plate 

 may be sensitised; especially Lehmann *) measured in this way 

 various lines with wave-lengths ranging to almost 1 (i. By means 

 of the bolometer Snow 3 ) could advance to '1.5 p. 



For the further region, however, nothing was known about these 

 spectra. Coblentz '), to be sure, was led by a series of observations 

 in this respect, to the conclusion that the alkali-metals emit no 

 specific radiation beyond 1.5;/, but I had reason to doubt the 

 validity of this conclusion. 



In what follows I will briefly describe the method by which some 

 ultra-red spectra were investigated, and the lines thus found. In an 



i) H. Lehmann. D.'s Ann. 5, 633, 1001. 



2) B. W. Snow. W.'s Ann. 47, 208, 1892. 



3) W. W. Coblentz. Investigations of Infra-red Spectra. Carnegie Inst. Washing- 

 ton. 1905. 



