234 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



it can be inspected through the plate-glass back of the cap P. In the 

 meantime the leak, pump, and coil have all been started to get the 

 bulb into the desired state. 



As soon as this is obtained and has become steady, J x is earthed to 

 prevent any rays reaching the camera when the plate is moved over 

 the slot to its first position, which is judged by inspection through P 

 with a nonactinic lamp. The magnet current having been set to the 

 particular value desired and the diaphragm adjusted, the coil is 

 momentarily interrupted while Jj is raised to the desired potential, 

 after which the exposure starts. During this, preferably both at the 

 beginning and the end, light from a lamp T is admitted for a few sec- 

 onds down the tube R (fig. 2), the ends of which are pierced with two 

 tiny circular holes. The lower hole is very close to the plate, so that 

 a circular dot or register spot is formed from which the measure- 

 ments of the lines may be made. 



The exposures may range from 20 seconds in the case of hydrogen 

 lines to 30 minutes or more, 15 minutes being usually enough. As 

 soon as it is complete the above procedure is repeated, and the plate 

 moved into the second position. In this way as many as six spectra 

 can be taken on one plate, after which L is shut, I 2 warmed up, and air 

 admitted to the camera. The cap P, which is on a ground joint, can 

 now be removed and the exposed plate seized and taken out with a 

 special pair of forceps. A fresh plate is now immediately put in, P 

 replaced, and the camera again exhausted, in which state it is left till 

 the next operation. * * * The accuracy claimed for the instru- 

 ment is about one part in a thousand. 



ORDER OF RESULTS AND NOMENCLATURE. 



The various elements studied will be considered as far as possible in 

 the order in which the experiments were performed. This order is 

 of considerable importance, as in most cases it was impossible to 

 eliminate any element used before the following one was introduced. 

 Evacuation and washing have little effect, as the gases appear to get 

 embedded in the surface of the discharge bulb and are only released 

 very gradually by subsequent discharge. 



The problem of nomenclature became serious when the very com- 

 plex nature of the heavy elements was apparent. After several pos- 

 sible systems had been discussed it was decided, for the present, to 

 adopt the rather clumsy but definite and elastic one of using the 

 chemical symbol of the mixed element with an index corresponding 

 to its mass — e. g., Ne 22 , Kr 84 . This system is made reasonable by the 

 fact that the masses of constituents of mixed elements have all so 

 far proved whole numbers on the scale used. 



In cases of particles carrying more than one charge it will be con- 

 venient to borrow the nomenclature of optics and refer to the lines 



