a A. Young— Duplicity of the "1474" 



430 



the dark one. Still there has never seemed to be sufficient ev- 

 idence to warrant a positive assertion. Examination with a 

 speculum metal " gitterplatte " of 6480 lines to the inch, some 

 two years ago, suggested the idea that the dark line might be 

 closely double, but the definition of the grating was not suffi- 

 ciently good to decide the question. With the new one, how- 

 ever, there remains no doubt. Another grating with 17280 

 lines to the inch, which is temporarily in my possession, shows 

 it nearly, though not quite as well. 



The accompanying map of this region of the spectrum gives 

 a good idea of the appearance of things, and the amount of dis- 

 persive power obtained. The scale numbers at the top are 

 those of Angstrom, those at the bottom are Kirchoff's. 



The observations are best ma( 

 order (with the coarser grating- . , , 



der with the grating of 17280 lines to tlie inch) although the 

 duplicity of the line is clearly visible in the spectra of the 6tti, 

 7th, and 9th order, with proper precautions. The arrangement 

 employed is that indicated in fig. 2. C is the collimator, the 



beam of light, thrown into the room by a heliostat, being c 

 centrated by the lens L which forms an image of the sun on 



