Scintillations produced by Radium. 429 



A small heap of the phosphorescent sulphide was pressed 

 flat with a clean piece of glass. A perfectly flat smooth sur- 

 face was thus formed, free from any cement or other binding 

 substance. A speck of radium was mounted above it, and 

 the scintillating surface photographed with a Zeiss microscope, 

 using a low-power objective, the magnification not exceeding 

 20 diameters. An exposure of three days was given, which 

 was sufficient to yield an excellent negative. A print from 

 the plate is reproduced in PI. VI. fig. 1. The photograph 

 reminds one strongly of a picture of the moon made with 

 telescope of moderate size. There are patches of considerable 

 size which only phosphoresce feebly, and small points which 

 shine brightly. The speck of radium was located at the 

 extreme edge of the field. Although the sulphide was in the 

 form of a very fine powder, the dark areas seemed to have a 

 considerable size, many times larger than the largest crystals. 

 The powder was rubbed for fifteen minutes in an agate 

 mortar and the experiment repeated. In this case the dark 

 areas were smaller and the bright specks more conspicuous 

 (fig. 2). Evidently certain crystals phosphoresced much 

 more brightly than others. To get a better idea of what was 

 going on, a glass slide was lightly dusted with the powder 

 and placed under the microscope. Two photographs were 

 made of the same field of view, one by illuminating the 

 particles with light (dark-ground illumination), and the ■ ther 

 by exposing them for three days to the radium bombardment. 

 The two pictures are reproduced in figs. 3 and 4. The arrows 

 point to the same spots on the two fields. It is at once 

 apparent that only a small percentage of the crystals become 

 luminous under the action of the radium rays. 



This may be due to the following circumstance. It is well 

 known that large crystals do not show the scintillations to 

 advantage. It is also a fact that the phosphorescent power 

 of most substances is due to minute traces of impurities. The 

 inert crystals are probably those in which the impurity causing 

 phosphorescence is not present, or if present, is buried under 

 the surface. This explanation of course assumes that the 

 crystals are not homogeneous, and we may suppose that the 

 flash of light occurs only when an electron strikes a molecule 

 of the impurity. Some crystals may not have the necessary 

 molecules in a position which can be reached by the electrons. 

 It would be worth while to pick out the active crystals and 

 photograph them in different positions, i. e. turning them over 

 between the exposures. They are so small that the experi- 

 ment would not be an easy one, but doubtless those who make 

 pictures out of diatoms would find no difficulty in accom- 



