46 Mr Purvis, The radiation of various spectral 



vibrating parallel to the lines of force, and two very diffuse ones 

 showing doubtful signs of division and vibrating perpendicularly to 

 the lines of force. The importance of this difficulty cannot be 

 exaggerated ; for when the gas is vibrating in the magnetic field, 

 its luminosity is greatly increased ; and the very faint images of 

 other orders, not clearly visible when the gas is not vibrating in the 

 field, would become better marked and clearer. The constituents 

 of these images would, therefore, interfere with a clear differentia- 

 tion of the constituents of any particular line under observation. 



The advantage of Lohmann's apparatus was the greater dis- 

 persive power of his echelon spectroscope of 32 plates as compared 

 with the one of 18 plates used in these experiments, and the 

 stronger field used in the present experiments did not appear to 

 compensate for the less dispersion of the echelon. So that until 

 the conditions of the experiments are more equal, no exact com- 

 parison of the observations can be made : and there is still the 

 more important difficulty of distinguishing the real constituents 

 of lines from the images of other orders. However, the distances 

 have been measured of the constituents of the lines unmistakably 

 divided into triplets, and, also, those of the line 6383 undoubtedly 

 divided into six constituents. 



The diameters of the capillary parts of the neon tubes were 

 different, so that the distances between the magnet poles had to 

 be altered to accommodate them : and the strengths of the field 

 were 24,000 and 26,100 units respectively. The following table 

 gives the distances of the two constituents of the triplets vibrating 

 perpendicular to the lines of force from the one vibrating parallel 

 thereto, measured on the scale of vibration numbers, i.e. the 

 number of vibrations in a path of one centimetre*. The positive 

 signs denote the distances in the direction of greater wave length, 

 and s and p that they vibrate perpendicular or parallel to the lines 

 of force. It may be of use to workers in this kind of research to 

 state that the analysing Nicol prism should be placed between the 

 magnet and the quartz condensing lens when analysing the various 

 constituents of a divided line ; for, if it is placed between the lens 

 and the slit of the spectroscope, the rotation of the polarised 

 constituents by the quartz lens produces a complete inversion of 

 the images when photographs are being taken in the ultra violet 

 part of the spectrum, so that the two outside constituents of a 

 triplet would appear to vibrate parallel to the lines of force, and 

 the middle one perpendicular thereto. It is possible that the 

 abnormal polarisations of certain triplets of various elements 

 which have been recorded have been produced in this way. 



* Eunge and Paschen, Sitz. d. Akad., Berlin, 1902 (1), p. 721, " Zeeman-effect 

 entsprechender Serienlinien." 



