680 Prof. R. W. Wood on Resonance Spectra of 



powerful echelons and plane-parallel plates, I am of the 

 opinion that the perfection of the grating, the circumstance 

 that it is very bright in the fourth order, and the long focus 

 of the lens, make it second to no other instrument in the 

 world capable of yielding a continuous spectrum. Lines in 

 the bromine absorption spectrum separated by a distance of 

 only '022 of an Angstrom unit are clearly separated in the 

 photographs, while the theoretical resolution is *018 A.u. 



The Absorption Spectrum. 



The absorption spectrum of iodine is made up of a large 

 number of fluted bands, and resembles in its general ap- 

 pearance the channelled absorption of sodium vapour, for in 

 both cases the bands at the long wave-length end are quite 

 regular in their appearance, while at the short wave-length 

 end they become more or less confused. As I have already 

 said, the number of absorption-lines which collectively form 

 the banded spectrum of iodine has been greatly under- 

 estimated. Sunlight from the heliostat was passed through 

 a large exhausted bulb containing a few small crystals of 

 iodine and focussed upon the slit of the instrument. The 

 absorption spectrum seen with tbe large spectrograph pre- 

 sented a most wonderful appearance, nearly the entire visible 

 spectrum being filled with thousands of lines. As I have 

 said, I found seven sharp and beautifully resolved lines 

 within the green emission-line of mercury. The total width 

 of the line was 0*4 a.e., and we have at this rate eighteen 

 lines to the Angstrom unit, or about 36,000 lines in all. 

 There were, however, groups containing lines much closer 

 together than the seven lines just enumerated, which were 

 still unresolved by the grating, and numerous broad dark 

 bands undoubtedly made up of unresolved lines. This cir- 

 cumstance, together with the fact that the lines are much 

 closer together in the red, orange, and yellow region, makes 

 me feel certain that there are upwards of 50,000 lines in this 

 remarkable absorption spectrum. 



The wave-lengths of the seven lines which were observed 

 within the green mercury line (furnished by the quartz 

 mercury arc) were very carefully measured with the eyepiece 

 micrometer, with reference to the wave-lengths of the com- 

 ponents of the mercury line seen with a low-temperature arc. 

 They were subsequently measured from photographs taken 

 with the 40-foot spectrograph, with reference to the main 

 component of the green mercury line, which bisects absorption- 

 lines 3 and 4, and the wave-length of which is 5460*7424. 



