1120 Prof. R, Whiddington on 



Method of taking Photographs. 



The film used was the Duplitized X-ray film of Messrs. 

 Kodak. A faint impression could be obtained with 5 minutes' 

 exposure from a copper foil, and a good photograph in 

 30 minutes. 



It was observed that the intensity of the photographs 

 obtained was not proportionately increased by longer 

 exposure, while at the same time the contrast became less 

 pronounced. Without further discussion, it may be o£ 

 interest to mention that excellent results were sometimes 

 obtained from these duplitized films by dividing the ex- 

 posure into two parts, one on each side of the film. It 

 was necessary, of course, to ensure that the film was placed 

 in exactly the same position relative to the camera during 

 each exposure, and this could be tested by the collimator Gr 

 of fig. 1. The lower photograph in PI. XXIV. was obtained 

 by this means. 



En order to test the resolving power, so to speak, of the 

 apparatus the chamber J was removed, the apparatus 

 exhausted with film in place, and a discharge passed from 

 an electrostatic influence machine working with no con- 

 denser. Under these conditions the cathode rays were 

 reflected from T through the slit into A and then bent 

 round to foci on F. The discharge was intermittent, so 

 that a velocity spectrum was produced giving the result 

 shown in the top reproduction in the Plate. It is clear 

 that the focussing is excellent, and that the individual 

 lines are sharply defined and practically as narrow as 

 the original slit *. 



The fundamental product pH. was measured from the 

 photographs obtained with an accuracy, it is believed, of 

 -§- per cent. It is to this accuracy, then, that the velocity 

 of the electrons corresponding to an intense well-marked 

 line can be guaranteed. 



Since, however, it is convenient to express the results 

 in terms of energy rather than velocity, it is proposed 

 to define an electron in "equivalent frequency" v, defined 

 by the usual relation hv = ^mv 2 i the relativity correction 

 having been introduced in calculating v from pIL = v .m/e. 

 It is clear, therefore, that v will be subject to a possible 



* Neglecting the relativity correction, -j- = ^ • »w/e ; where p is the 



radius of curvature of an electron velocity v in field H. It can easily 

 be shown from this that the " dispersion " and " resolving power " for 

 the apparatus is nearly independent of v — or the spectrum is normal 

 for a given value of H. 



