R. M. Field — Roentgen Ray in Paleontology. 547 



penetrate specimens of such rocks as shale, sandstone and 

 limestone at least 2 centimeters thick. It was also found upon 

 examining the skiagraphs that the surface inequalities of the 

 specimens had been faithfully reproduced. This discovery 

 led to experiments with rocks which bore merely the print of 

 an organism, such as the moulds of striated brachiopods and 

 of the cephalon and pleura of trilobites in a sandstone. As 

 had been expected from the first experiment, the resulting 

 skiagraphs showed the surface details almost as clearly as if 

 they had been photographed. Having determined that the ray 

 would penetrate and record upon the X-ray plate the graphs of 

 the common sedimentary rocks up to at least 2 cm in thickness, 

 the next problem was to determine if the X-ray would bring out 

 on the X-ray plate some petrified organism which was partially 

 exposed on the surface of a sedimentary rock. It had already 

 been discovered that inequality of surface was skiagraphed upon 

 the X-ray plate by the penetrating ray. It was therefore natural 

 to suppose that the slightly elevated form of any fossil pre- 

 served upon the surface of a thin section of rock would be 

 skiagraped upon the plate when subjected to the penetrating 

 ray, no matter if the fossil itself were composed of the same 

 material as the rock which partly enveloped it. But what 

 would be the fate of that portion of the fossil which remained 

 hidden ? Experiment shows that it will be registered when 

 the fossil is composed of a substance whose density is greater 

 or less than that of the matrix in which it lies. Physicists agree 

 that theoretically the ray penetrates with an intensity which is 

 indirectly proportional to the atomic weight of the substance 

 treated. This fact has long been recognized by users of the 

 Roentgen apparatus who protect themselves from the ray by 

 means of a thin lead screen (see table on previous page). In 

 the early days of experimentation with the X-ray no protection 

 was afforded the eyes of the operator from the rays, which 

 emanated from the tube in all directions. Recently it has 

 been discovered that a shield made of glass which has a high 

 percentage of lead added to the melt during its manufacture 

 partially prevents the escape of the rays except within the 

 limited direction required. 



In the latter case we have a solid solution of nearly the same 

 transparency as ordinary glass but which has had its density 

 increased by the addition of an element of high atomic weight, 

 thus rendering it much more impenetrable to the ray. (It is 

 not certain whether the lead oxide is held in solution or sus- 

 pension.) Bearing these facts in mind we should expect to 

 discover the clearest definition of structure on the skiagraph 

 when the mass to be penetrated is made up of compounds 

 whose elements show the greatest differences in atomic weights. 



