456 JOHN W. GOWEN 



the use of such X-ray equipment as is often found in dental chnics, 

 where it is intended for intermittent operation only, with separate 

 exposures limited to a matter of seconds. 



The protection of the operator from the X rays generated by the 

 X-ray machine is of first importance. Lead shields are normally 

 provided for this purpose. It is well to read the bulletins of the 

 Bureau of Standards on suitable protection of the operator from X- 

 ray effects {55). 



B. DOSIMETRY 



1. Units of Measurement 



Since the discovery of X rays, the measurement of their intensity 

 has been conducted along two quite different lines. For biological 

 measurements the unit has changed as increasing information became 

 available on the biological effects of this energy. One of the first 

 noticed effects of X rays was an erythema or a reddening of the skin, 

 occurring either immediately or as a delayed reaction appearing 

 after some days or weeks. To the clinician this effect was obviously 

 the beginning of cell changes that weie rather serious to the tissues. 

 The "erythema dose," defined as the minimum X radiation to pro- 

 duce an erythema after ten to fourteen days, became a biological 

 measure of X-ray dosage. It suffered the disadvantages that people 

 varied in their susceptibility to radiation and that the unit differed 

 according to whether radiation was delivered to the skin continuously 

 or intermittently. 



A second biological unit used for X-iay measurements is the sur- 

 vival of such objects as Drosophila eggs. These survival tests were 

 found to be remarkably constant in eggs from rapidly laying Dro- 

 sophila exposed two hours after laying. The unit was the X radia- 

 tion necessary to cause the death of 50% of the eggs exposed under 

 standard conditions (43). The biological measures of X radiation 

 are handicapped by lack of trained personnel and easily available 

 material, as well as from the time elapsed between the radiation ex- 

 posure and a knowledge of the dose administered. 



The physical measurement of X radiation, on the other hand, can 

 be made in a very few minutes with easily standardized equipment. 

 Until the biological and physical methods were brought into agree- 

 ment, however, no common ground for the use of one method over the 



