[cox A callendar] experiments ON THE X-RAYS 175 



inch gap in air once in every live or ten discharges. It was not possible 

 to keep the proportion quite constant, but a verj- fair average was main- 

 tained. A little air was admitted ever}" one or two minutes. The 

 character of the discharge, and the X-radiation appeared to be nearly 

 unchanged throughout. The intensity was, if anything, greater at the 

 end of the exposure. At the end of the ex])Osure, however, the pressure 

 of the air in the tube was found to have increased to nearly the same 

 extent as if the air admitted to the tube had all ace mulated in the tube, 

 and had not been used up by the discharge or occluded by the walls as is 

 generally supposed. This would appear to imply that the production of 

 kathode rays is due in part at least to some change in the constitution of 

 the gas in the tube, and is not merely a question of the degree of vacuum, 



After allowing the tiibe to rest for three hours, the vacuum was 

 found to be almost unchanged, but no kathode rays were produced until 

 the discharge had been passed for nearly a quarter of an hour. Similar 

 phenomena were observed on other occasions after prolonged exposures. 

 It is possible that the apparent increase of pressure may have been really 

 due to the removal or decomposition of aqueous vapour by the discharge, 

 but we could not find any direct evidence that this was the case. A 

 similar increase of apparent resistance is observable in most tubes during 

 the first few minutes of the passage of the discharge. In manj^ cases 

 the resistance ceases to increase after a short time, and the tubes will run 

 continuously without change for half an hour or more. In the case of 

 sealed tubes, if air-tight, the original state of the vacuum may be 

 restored by heating, or by an alternating current. 



Anatomical Photographs. — With the focus tube, there is now no 

 difficulty in taking photographs of any of the joints or extremities, which 

 are capable of showing the nature of any injury due to fracture or dislo- 

 cation or the presence of foreign bodies, just as clearly as if the bones 

 themselves Avere exposed to view. In taking photographs through the 

 thicker and more solid parts of the trunk, however, there are still ditfi- 

 culties, owing to the fact that the bones appear to be much less opaque 

 as compared with the flesh when tested by rays of sufficient intensity to 

 pass through a considerable thickness. Plenty of light gets through, 

 and it is possible to obtain extremely dense negatives with an exposure 

 of ten or fifteen minutes, but it appears that the rays undergo a kind of 

 filtering process in passing through the upjier layers, and that those 

 which survive extinction longest, penetrate bone and flesh alike with 

 more nearly equal facility. It is fortunate, however, that bullets, buttons, 

 and other metallic objects, are so much more opaque than bone or flesh 

 as to be very readily distinguished in any part of the body. 



It is probable that many improvements remain to be made in this 

 direction by the use of suitable fluorescent screens in conjunction with 

 the ])hotographic plate, or of suitably stained or loaded emulsions. 



