Iiiet-tia. 



Contrast. 



Fog- 

 density 



1.17 X I0-« 



- 2.4 



- .59 



.74 



- 2.3 



- .23 



.71 



- 2.2 



- .30 



1.00 



- 2.3 



- .60 



1.12 



- 1.9 - 



- .45 



1.95 



2.2 



.51 



1.70 



- 2.0 



- .24 



4.68 



- 3.1 



.40 



1.26 



- 1.6 



- 1.44 



2.14 



- 1.9 



.55 



2.19 



- 1.9 



- .36 



1.44 



1.5 



- .18 



.81 



- 1.3 



- .38 



1.58 



1.4 



- .27 



3.71 



J. 2 



- .35 



1.23 



- 1.2 



- .59 



Sensitiveness of Photographic Plates to X-rays. 427 



Plate. 



1 Paragon X-ray - - - 



2 Paragon X-ray - 



3 Diagnostic X-ray 



4 Simic X-ray . . - 



5 Seed X-ray - - 



6 Wratteii X-ray 



7 VV^elliiigton X-ray 



8 Austral X-ray - - - 



9 Imperial X-ray 



10 Cramer X-ray - 



11 Ilford X-ray - 



12 Imperial Special Eapid 



13 Imperial Special Sensitive 



14 Austral Standard (Sun) 



15 Austral Standard (Extra Eapid) 

 J6 Austral Standard (Extra Rapid) IE. 



Although all the Paragon X-ray plates used were from the same 

 batch, two distinct exposure lines were obtained, both giving the 

 same contrast, though the inertia was much greater in one case 

 than in the other. Apparently, some of the plates had received a 

 thicker coating of emulsion than others. 



Salomonsoni found that, while the inertia of any given type of 

 plate was always the same, whatever the hardness of the rays, the 

 slope of the exposure line and, therefore, the contrast of tlie plate, 

 was greater for the more penetrating radiation. Hodgson, 2 on 

 the other hand, found in his measurements of the density of 

 Seed X-ray plates, that the contrast diminished with the increasing 

 hardness of the radiation. In this work, however, the exposure 

 lines for the three different pressures, th,at- is for three different 

 wave lengths, were found to be coincident. The inequalities in the 

 thickness of the emulsion on the plates are more than sufficient to 

 account for any departures from the straight line. The actual 

 points obtained with the Diagnostic X-ray plate for each of the 

 three pressures used are shown in the graph Fig. 3 ; the three 

 exposure lines are coincident within the limits of accuracy possil)le. 



The accuracy of the results is limited by the variations in the 

 thickness of the emulsion on the plate. In some cases variations 

 even in the small strips being photometered were noticeable. In 

 order to find the magnitude of errors due to this cause the follow- 

 ing tests were raade:^ 



(1) An unexposed Wratten X-ray plate (half-plate size) was 

 developed and the fog-density was measured at eight different 

 points. Ihe values obtained were :— 1.62, 1.5G, 1.62, 1.83, 1.82, 



1 Saloinonson. Loc. cifc. 



2 Hod>,'Tion. Loc. cit. 



