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ANNUAL REPOBT SMITHSONIAN INSTITUTION, 1910. 



diminish in their power to make the screen fluoresce as the square 

 of the distance from it. You will see on the screen the character- 

 istic curve of a selected selenium cell for feeble illumination, the 

 maximum being of about the same wave length as that of the fluores- 

 cence, showing the relation between resistance and distance separating 

 the source of illumination and the cell, and also the modified curve 

 showing a similar relation between resistance and distance between 

 antikathode and cell, with the screen in contact. The portion of the 

 first curve most nearly asymptotic is best to employ for the work, 

 and from the second curve the dial scale of the meter can be easily 

 calibrated. If, now, I vary the height of the X-ray tube from the 

 measuring apparatus, you will see that the meter needle is deflected 

 less as the distance between tube and cell is increased. The actual 

 instrument is provided with a scale divided so as to show compara- 

 tive times of exposure, and by its use radiographic work can be 

 greatly facilitated. 



It is interesting to note that the effect of the rays on the fluorescent 

 screen, as estimated by the selenium cell, differs less with increasing 

 distance the farther the antikathode is from it : 



A good deal of time has, I am afraid, been taken up in giving de- 

 tails of apparatus, but I will now show some of the results that have 

 been obtained in practice. The selenium machines already referred 

 to were operated between Paris, Manchester, and London until the 

 end of the year 1908. The first photograph received (slide) was of 

 King Edward, and was received at the Daily Mirror installation in 

 November, 1907. Several results will now be shown in the lantern, 

 and you will observe that they are all composed of parallel lines, 

 which widen or " thin " according to the density of the picture. 

 These lines correspond to the movement of the shutter attached to the 

 strings of the Einthoven galvanometer, which regulates the thick- 

 ness of the spot of light focused on the revolving sensitive film. This 

 spot of light* traces a spiral line around the film, which, when de- 

 veloped, is laid flat, and the spiral becomes resolved into so many 

 parallel lines. 



