Mass-Spectra of Chemical Elements. 



617 



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 measurements 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. 



Form of the Spectrum Lines. 



As has been shown (Phil. Mag. Dec. 1919,, plate ix.),the 

 shape of the spot formed when undeflected rays from such a 

 slit system strike a photograph surface normally, is somewhat 

 as indicated at a (fig. 4). When they strike the plate obliquely 

 the image would be spread out in one direction, as in b. 

 This would be the actual form in the apparatus, if the 



Fia-. 4. 



V 



deflexions of the mean and extreme rays (i. <?., the rays 

 forming the centre and the tips) were identical. This is 

 true of the magnetic field since each cuts the same number 

 of lines of force ; but it is not so in the case of the electric 

 deflexion. Since the form of the plates, and therefore 

 roughly of the boundaries of the field, is rectangular, the 

 extreme rays passing diagonally will be deflected more than 

 the mean rays and the spot bent into the form shown at c. 

 The convex side will be in the direction of the magnetic 

 deflexion, as this is opposed to the deflexion causing the 

 Phil. Mag. S. 6. Vol. 39. No. 233. May 1920. 2 S 



