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



as well as because of certain results obtained through the use 

 of newer and hence more powerful and accurate apparatus, 

 that the writer is able to add something to the previous work 

 on this subject. None of the writer's investigations could have 

 been possible had it not been for the kindness, generosity and 

 skill of Dr. Walter James Dodd, who for years has devoted 

 his energies to the development and manipulation of the 

 Roentgen apparatus. About thirty skiagraphs were taken at 

 Dr. Dodd's laboratory. All the fossils used in the experi- 

 ments were selected from the collections belonging to the 

 Laboratory of Paleontology at Harvard University. 



Technique. 

 The writer does not feel justified in entering into a discus- 

 sion of the general methods of skiagraphy. The subject is 

 complex and its practice requires the knowledge and experi- 

 ence of the expert who has learned to manipulate his appara- 

 tus. Much remains to be learned regarding the best methods 

 for the skiagraphy of inorganic substances and experiments 

 carried on by the paleontologist in connection with the physic- 

 ist should yield the best results. As stated elsewhere in this 

 paper, the best skiagraphs can only be obtained by one having 

 an intimate knowledge of the nature of the petrifaction. The 

 physicist knows the limits and possibilities of his apparatus 

 and the paleontologist is needed to select the material for the 

 experiments and to interpret the results. Working together, 

 the physicist and the paleontologist should be able to develop 

 the technique of the skiagraphy of fossils just as the surgeon 

 and the physicist have worked such wonders in skiagraphy of 

 the human tissues. Any apparatus that will give a 6 to 10 

 inch spark can be used for skiagraphing fossils. In making 

 the plates used for illustrating this article the penetration was 

 raised from 7 to 10 Benoist, which is equivalent to from 70,000 

 to 100,000 volts and 50 milleamperes. Experiments showed 

 that for specimens of low density (comparative) more contrasty 

 skiagraphs could be obtained by using an intensifying screen 

 and lower voltage, say 50,000 to 60,000, the exposure in this 

 case being \ second. No exposures were made of over one 

 second duration. The special photographic plates which are 

 used for organic investigation were found to work equally well 

 for the skiagraphy of fossils. When placing the fossil on the 

 plate care should be exercised to have the structures to be dis- 

 covered nearest the plate and farthest from the source of the 

 rays. 



Experiments in Skiagraphy of Fossils. 



In the preliminary experiments it was discovered that a 

 pencil of rays having an intensity of 6 Benoist could easily 



