Rontgen Ray Photographs. 245 



the central portion of this layer was covered with thirty-two 

 additional layers of the same foil about 2*8 cm square. Plate 

 IX, fig. 1, shows the result when the plate was exposed for 2*5 

 minutes ; fig. 2, when exposed for 5 minutes, and Plate X, 

 fig. 1, when exposed for 15 minutes. These photographs 

 show that with a short exposure, the portion of the plate 

 covered with a single layer of tinfoil was less affected than the 

 uncovered portion of the plate. When the time of exposure 

 was increased, the shadow of the single layer of tinfoil would 

 only be noticed on the negative by careful inspection, and 

 might easily escape detection. In the case of the longest expos- 

 ure, where the portion of the plate covered with the single 

 layer of the tinfoil is most affected, we have a reversal of the 

 image and a clear case of solarization. 



Experiments were also made with photographic plates par- 

 tially covered with an aluminum cone having an altitude of 

 l*25 cin and a base 5 cm in diameter. When the exposure was 

 short, the uncovered portion of the plate was most affected. 

 When the plate was placed at a distance of 15 cm and exposed for 

 5 minutes, the effect reproduced in Plate X, fig. 2, was obtained. 

 In this case, the portion of the plate under the edge of the cone 

 was most affected. When the time of exposure was increased 

 to 15 minutes, all of the plate covered by the cone was affected 

 more than the uncovered portion, and again we have a rever- 

 sal of the image and a clear case of solarization. 



Seed, Kramer crown, and Carbutts' special X-ray plates, and 

 various developers were tried and found to give similar results. 

 Carbutts' X-ray plates and Carbutts' tabloids for developer 

 were used in making the photographs for the accompanying 

 illustrations. 



These experiments seem to prove conclusively the possibility 

 of photographic plates becoming solarized by Rontgen rays. 

 This is interesting as adding one more to the properties 

 possessed in common by these rays and ordinary light. Solari- 

 zation offers a simple explanation of many of the halo effects 

 observed in Rontgen ray photographs. The most important 

 conclusion to be derived from these experiments is the neces- 

 sity of carefully timing exposures if we are to obtain good con- 

 trasts, much of the indistinctness in Rontgen ray photographs 

 being due to over-exposure rather than to under-exposure. 



I desire to express my obligation to Columbia University 

 for the very material assistance given me in carrying on these 

 experiments by placing at my disposal the income of the 

 Barnard Fellowship. 



Jarvis Physical Laboratory, Trinity College, 

 Hartford, Conn., July 24th, 1897. 



Am. Jour. Sci.— Fourth Series, Vol. IV, No. 21.— Sept., 1897. 

 17 



