202 J. Stanley Rogers: 



5. and lO.min., and, at the end of this time, the tube would be 

 again in a condition suitable for use. To obtain lines sufficiently 

 intense to measure, an exposure of 10,800, niilliampere sec. was 

 necessary. Since the average current through the tube was be- 

 tween .7. and .8.m.a., such an exposure required an experiment of 

 at least six hours. The greater intensity of the radiation from 

 a Coolidge tube was shown by the fact that more intense lines 

 were obtainable from it in S.min. 



The film used was Eastman Dulpli-tized (photographic emul- 

 sion on both sides) which was placed between two Patterson 

 intensifying screens. It has been stated that sharply defined lines 

 cannot be obtained when intensifying screens are used. How- 

 ever, quite satisfactory lines were obtained, since under these 

 conditions the a doublet of tungsten was resolved with a slit 

 width of 0.12.mm. One great advantage of the doubly-coated 

 film is its rigidity — there is no tendency to buckle during drying. 

 Had intensifying screens not been used, the exposures would 

 liave been so long as to be practically impossible. 



Protection from Scattered Radiation. — A sheet of aluminium 

 •0.6.mm. thick was placed in front of the film to absorb the soft 

 scattered radiation. In addition the rays reflected from the crys- 

 tal were made to pass along a channel the sides of which were 

 constructed of lead 3.mm. thick. This channel converged on all 

 ^ides towards the crystal where its opening was a rectangle 2.cm. 

 X 6.mm. This was large enough to allow both the direct rays 

 and the lines to fall on the film. 



Reference Lines. — The Pt. lines were photographed on the top 

 half of each film, the lower part being covered with a lead screen. 

 The Coolidge tube was then substituted for the Gundelach, and 

 the W lines were photographed on the lower portion of the 

 film. Since the film was held by the red fibre strips (vide 

 page 198), there was no opportunity for it to slip during an ex- 

 periment. The lines appeared as shown in figure 2. It will be 

 seen that the a doublet of platinum falls between the a and yS 

 lines of tungsten. (The thickness of lines in the figure indicates 

 relative intensities.) 



Measurement of Lines. — The film was projected by a lan- 

 tern, and a magnification up to 10 was obtained. The lines on 

 each film were measured at three different magnifications, and 

 the mean of the values so obtained was taken as the wave lengths 

 of the lines given by the film. 



