372 Prol:. Wood and 31r. Moore on the Fluorescence 



slit-width of 0'065 mm. The width o£ the slit for the iron 

 comparison-spectrum was 0*030 mm. In order to eliminate 

 any errors due to changing the width of the slit, photographs 

 were taken of both spectra with a slit- width of 0*030 mm., 

 and the measurements made with the two sets of plates com- 

 pared. The plates were measured on the dividing-engine of 

 the laboratory, and the measurements can be considered 

 accurate to Avithin 0*05 of an Angstrom unit. The ultra- 

 ^•iolet region was explored in the same manner, using a tube 

 closed with quartz plates. Xo trace of any flutings Avas 

 found, but the lines of the principal series were strongly 

 reversed. 



The fluted absorption-spectrum makes its first appearance 

 when the D-lines are two or three Angstrom units in width. 

 It begins as nine bands, the heads of which point towards 

 the violet end of the spectrum, i. e. in each band the absorp- 

 tion decreases from the head towards the longer wave-lengths. 

 The heads of the bands as they first appear are : — 



1 4783-30, 



II -1809-92, 



III 4837-72, 



IV 4865-60, 



V 4894-94, 



VI 4932-97, 



VII 4962-96, 



VIII 5001-94, 



IX 5040-71. 



There is also evidence of absorption-lines in the first seven 

 bands : these lines, however, do not come out strong at this 

 stage, but appear as a slight shading on the bright back- 

 ground. In band I. can be seen the line 4793-10, in band II. 

 line 4820-72, in band IX. 5053*50, and in band VII. 4979-28, 

 which appear as fainter heads of small bands into which the 

 others break up. Bands V. and VI. are at about the centre of 

 the region of absorption. (Fig. 3, Plate XIV.. iron comparison- 

 spectrum.) 



A very slight increase in the density of the vapour is 

 sufficient to bring out the heads of the bands as strong dark 

 lines, as well as the fainter heads in the bands themselves. 

 The bands are now seen to be made up of an immense number 

 of lines, some of which are broad and some exceedingly fine, 

 and in some of the bands there is an appearance as if two 

 series were superposed. (Fig- 4, Plate XIA^.) 



At this time the region of absorption extends from about 

 4600 to 5200, a series of three bands, the more refrangible 



