W. L. Robb — Solarization Effects, etc. 243 



Art. XX V I. — Solarization effects in Rontgen Ray Photo- 

 graphs ; by Wm. Lispenakd Kobb. (With Plates VIII-X.) 



It has long been well known, that in photographing with 

 ordinary light, in cases of over-exposure, the picture upon 

 development may be a positive instead of a negative. This 

 phenomenon is known as solarization, as it is usually produced 

 by over-exposure in strong sunlight. Also, in case of over- 

 exposures not sufficiently long to produce a reversal of the 

 .image, the photographic plate may be so affected as to prevent 

 satisfactory development. 



Some experiments that I have recently made show that sim- 

 ilar effects are produced by the Rontgen rays, and that they 

 have a very important bearing upon the distinctness of photo- 

 graphs taken with these rays, and offer a very simple explana- 

 tion of the halos that so often appear in such photographs. 



The following apparatus was used in these experiments : 

 The induction coil used was a " Thompson Inductorium " as 

 manufactured by the General Electric Co., except that a rotary 

 break was substituted for the one furnished with the coil. The 

 rotary break consisted of a solid brass ring 25 cm in diameter 

 and 5 cm thick. Two slate quadrants, 2'5 cm thick, were counter- 

 sunk in the ring. Two copper brushes were arranged so that 

 one was always in contact with the brass and the other alter- 

 nately with the brass and slate sectors. The ring was mounted 

 on the shaft of a 1 h. p. motor, making 1800 revolutions per 

 minute, and consequently the primary circuit of the induction 

 coil was made and broken 3600 times per minute. A con- 

 denser having a capacity of six microfarads was connected 

 with the two brushes. This was the largest condenser available ; 

 and it was found that the sparking at the brush, where the cir- 

 cuit was alternately made and broken, was very great, being 

 sufficient to cause great unsteadiness in the illumination of the 

 screen of a fluoroscope. A very simple method was found for 

 overcoming this unsteadiness. A third brush made of mica 

 was placed so as to be in contact with the break and to form 

 an obtuse angle with the brush at which the sparking occurred. 

 This additional brush prevented the sparking ; and after it was 

 adopted, the illumination of the fluorescent screen was entirely 

 free from flickering. The current in the primary was adjusted 

 in the following experiments so that the coil would give a 

 spark 25 cm long. Single focus vacuum tubes were used. The 

 tubes were spherical in form, about 14 cm in diameter, and the 

 distance between the anode and cathode was about 8 cm . The 

 tubes used were made by Green & Bauer of Hartford, Con- 



