ON THE ULTRA-VIOLET SPECTRA OP THE ELEMENTS. 



Again, it seldom happens that a grating gives the two spectra of the same order 

 equally bright. With one of our gratings the spectra of the 3rd and 5th orders are 

 bright on one side only of the normal, while those of the 4th and 6th orders are 

 bright on the other side only. Where faint lines were in question, therefore, we have 

 used spectra of different orders on the two sides, and computed the wave-length by the 

 formula (m + n)\ = 4a sin (a -f ft) cos (a ft) ; where ra and n are the orders of the 

 spectra employed, a and ft the deviations of the ray from the normal, (a -f- ft) is, of 

 course, the angle directly measured ; (a ft) has to be calculated from the reading of 

 the instrument when the grating is normal to the axis of the collimator. As (a ft) 

 is always a small angle, any error in determining it will affect the value found for X 

 but little. 



Measures of other cadmium lines gave the following results : 



Previous measures of lines Nos. 24 and 25, by interpolation between the copper 

 lines of which the wave-lengths had been determined by RUTHERFORD'S grating, had 

 given the values of X 2265'0 and 2194'2. 



Other cadmium lines, measured by ROWLAND'S grating, gave the following results : 



M DCCCLX X X VIII. A. 



2 H 



