Magneto-optics of Iodine Vapour, 1011 



the rays o£ absorption, which' are extremely fine and very 

 close together, over one hundred having been counted in 

 region 6 A. XL in width (distance between the D lines), on a 

 photograph made with the 42-foot plane grating spectrograph 

 at East Hampton, N.Y. 



The remarkable resonance spectra excited when the vapour 

 is stimulated by monochromatic light of a frequency corre- 

 sponding to that of one of these very fine absorption lines, 

 have shown the importance of a study of the vapour in a 

 magnetic field with a resolving power sufficient to clearly 

 separate all of the lines. 



The aim of the present investigation has been to determine 

 the exact nature of the rotation produced by the lines of absorp- 

 tion, since the more recent investigations, just alluded to, have 

 shown that the earlier results dealt with rotations produced 

 by close groups of lines, no record having been obtained of 

 the nature of the rotation to the right and left of a single line. 

 It was of especial importance to determine whether the 

 rotation to the right and left of an absorption line was of 

 the same nature, i. e. either positive or negative, as is the 

 case with the D lines of sodium, or whether any case of 

 anomalous rotation occurred, i. e. positive on one side of the 

 line and negative on the other. 



Small glass bulbs about 2*5 cm. in diameter, highly 

 exhausted and containing a crystal of iodine, were mounted 

 between the poles of a large Weiss electromagnet. The 

 bulbs were supported in a brass tube of 3 cm. internal 

 diameter, furnished with two lateral holes for the passage of 

 the light, and heated electrically by a spiral of nickel wire 

 placed below the bulb. A cover of mica 

 forced the heated air rising around the bulb 

 to escape through the side holes, and pre- 

 vented iodine crystals from depositing on 

 the walls in the path of the beam of light, 

 as was invariably the case if the brass tube 

 was open at the top. The source of light 

 was a quartz mercury arc arranged " end on," 

 the observations being restricted to the seven 

 or more absorption lines of iodine which are 

 covered by the broadened green mercury 

 line. 



Our first observations were made with a six-inch plane 

 grating in the fourth order spectrum (res. power 300,000) 

 combined with a plano-convex lens of 4*20 m. focus. This 

 lens was kindly placed at our disposal by M. Deslandres, 

 director of the Meudon Observatory. 



